Authors: Isis (isiscolo, website HieroglyFICS) and Fabula Rasa (fabularasa, website Fabulae). Fab wrote the "dynamic" section, we each wrote part of the "characters" section, and Isis wrote the rest.
Spoilers: Prisoner of Azkaban (PoA), Order of the Phoenix (OotP)
Thanks to cordelia_v, arachnethe2, trobadora, and themostepotente for resources and suggestions.
The pairing of Severus Snape and Sirius Black is classic "enemies slash." These two men, arguably the most complex and compelling characters in the Harry Potter series, have a long history of antagonism and opposition - so naturally, they're incredibly hot together. No, actually what I meant to say is that their shared history provides rich fodder for plotty stories, and the canonical tension between them just cries out for explanation and resolution. And sex, of course.
One of the reasons Rowling's books fly off the shelves faster than flannel sheets at a white sale is her ability to inject nuance into a seemingly straightforward bildungsroman. On the one hand we have Harry, the Good Guy; on the other, we have Voldemort, the gleefully revolting ubervillain. Good versus evil, light versus dark; the story of every fairy tale since Gretel matched wits against the witch.
But against this framework, some vastly more interesting things are happening. There are adults in these books, for one thing – real live functioning adults, and for those who find adults and all their complicated baggage just sexier than teenagers, they will remain the primary focus of the books. Plenty of these adults, to be sure, are little more than two-dimensional caricatures; Vernon Dursley and Lucius Malfoy are two of the most unconflictedly evil, and Arthur and Molly Weasley (the Mike and Carol Brady of the piece) are so aw-shucks sweet your teeth rot just reading them. But sprinkled among them are two fascinating exceptions: Severus Snape and Sirius Black.
Severus Snape is described as a thin man with a hooked nose and greasy black shoulder-length hair. Despite this, he's probably the most frequently-appearing adult character in pairing-oriented fic, possibly due to Alan Rickman's yumminess in the movies. Sirius Black was once handsome, but is now gaunt and weary-looking; he also has long black hair.
The perceptive reader will at once notice the physical similarities between the men, the distorted mirror image one is to the other, and of course this metaphor can be applied to far more than the physical. They were at Hogwarts together in rival houses: Snape in Slytherin, Sirius in Gryffindor. (And isn't it interesting that generally fans refer to one by surname and one by first name?) Both appear to have had uneasy relationships with their families, although Snape was most likely from a poor or lower-class family, while the Blacks were wealthy. Sirius had broken with his Voldemort-supporting family; Snape aspires to everything Sirius is turning his back on.
Snape had been a Death Eater in his youth, then became a member of the Order of the Phoenix, spying for them "at great risk to his life," according to Albus Dumbledore. This redemptive path is reversed in Black's early alliance with the Order, followed by his apparent betrayal of the Potters and murder of Peter Pettigrew, for which he was wrongly imprisoned in Azkaban. (But of course he turns out to be a good guy in the end.)
Snape, as we have him in canon (and not in the thousands of drool-worthy fanfics celebrating his dark mystique) is not a nice guy. He is, in fact, a complete asshole. He is the dramatic focus of the first book, as young Harry encounters him, is made miserable by him, and then assumes that Snape, because he is unpleasant and mean, must also be evil. He makes the classic mistake of confusing good with nice, and the book's genius lies in its deft destruction of youth's comfortable assumptions. Right away Rowling lets us know that in this showdown of Good versus Evil, we are going to be allied with people we may not like and who may not like us.
And then, we have Sirius, the guy Harry (and, to be fair, the entire wizarding world) assumes to be Evil, but is actually a Good Guy. Only, the picture is a bit more complicated than that. Rowling hints around it in Prisoner of Azkaban and Goblet of Fire, but in Order of the Phoenix we see for ourselves: Sirius is not such a nice guy after all. In fact, he's a bit nasty and unpleasant himself, when he wants to be. After all, when confronted with his teenage bullying, he doesn't exactly hang his head in shame and explain to Harry that people change and mature, and how sorry he is to have been so mean, etc. etc. He comes across as unstable, impetuous, and given to occasional meanness.
Both Snape and Black are flawed, to say the least. But they are interesting to a reader precisely because they are larger than their flaws, and because they recognize, through their own smudged lenses, where Good resides. They are conflicted guys, often petty and childish guys who struggle with their loyalty to something better than themselves. Conflict – their internal conflict, and their external conflict with each other -- is what draws a reader in. It is also an irresistible ingredient in writing fiction. Who knows what such unpredictable and volatile individuals are likely to do? It's like a junior chemistry set where the instructions say no matter what, don't put liquid A in with compound B, and of course, that's the first thing you do. Cuz look, Ma! Explosions!
Something else interesting is going on with Snape and Black, too. To a degree, they can be seen as reflections of the conflict between Harry and Voldemort. Because this is a fairy tale, Harry and Voldemort are drained of nuance and inner conflict. All that nuance gets transposed onto those around them. Snape is a surrogate for Voldemort in Harry's life. Who, after all, represents the real day-to-day problem in Harry's life? Not scaly old Voldemort, who slinks infrequently onto the scene and then slithers away. It is Snape who is the Daily Evil with which he must contend, the workaday, routine Evil of his world. And who is Sirius but his own (as he sees it) better self and ideal, the one he in so many ways yearns to be like – so handsome, so devil-may-care, such the dashing Gryffindor. So in a way, the conflict between Sirius and Snape is the conflict between Harry and Voldemort writ small – or at least, writ more interesting, writ more subtly, writ in a way calculated to appeal more to the adult reader. And who, in that conflict, is the Good Guy, and who is the Evil Guy? Fanon is full of speculation, naturally – Snape has his impassioned defenders, as does Sirius.
One of the things Rowling has yet to reveal to her readers is the why of their conflict. Is it simple bullying by the strong of the weak? Or is there some other reason, some as-yet-unknown first cause for the way Sirius and his friends treat Snape? One of the reasons I got drawn into writing Snape/Black is that it invites creating an answer to that question, and in my heart of hearts I suspect their enmity, their ongoing (ongoing if you can overlook Sirius' death, or if you suspect, as I do, that we've not seen the last of him) conflict may be at the root of the plot. We know the facts: this particular group of friends teased and victimized Snape, for reasons unknown, culminating in the Shrieking Shack incident in which Sirius lured Snape to be attacked by Lupin during his full-moon change into a werewolf; and then one of those friends turned traitor and caused the death of another of the friends, and then Snape ends up on the same side as the traitor, and then turns back, we assume. Are all these facts connected? How? What could be the thread? The richness of possibilities is one of the things that drives any writer of Snape/Black. Apart from, you know, the hotness of two powerful angry men going at each other, in every sense of the phrase.
Writing "enemy slash" is, for a writer, the double-peaked mountain; it's the thrill of the challenge. Not only do you have to get your characters to overcome the "dear God I'm having gay sex!" hurdle, you also have to get them over the "and it's with you!" hurdle. How do you handle that double challenge, as a writer? Can you keep your reader believing it? More than that, can you convince your reader that this is the only way it can be, that this is the logical and necessary conclusion? It's sleight-of-both-hands, and while it can be spectacular if done well, it can also be a dismal failure in the wrong hands.
Then of course, there is the whole "I hate my enemy because I am so very like him in so many ways." Snape and Black, for all that they like to pretend they are polar opposites, are not so far apart. But beyond similar personal histories and similar appearances, there is their similar personality: both overly quick to make judgments and stick with them, both overly quick with their temper, both able to hold onto a grudge long after everyone else in the room has forgotten what the fuss was about. As I said – not very nice people. But I'm the kind of girl that always wants to get to know the not very nice people, which isn't so much a kink as a personality flaw of my own, I suppose.
But I think that Snape/Black is a neglected corner of the fandom, certainly. When I first read the books I was completely convinced the whole thing was a parable of tortured homosexual psychomachia centered on these two men, and so I got on the net thinking I was going to find it littered with Snape/Black this and Snape/Black that. Didn't find it. I found a mountain of Harry/Draco, though, and an equally large mountain of Snape/Harry, which made me scratch my head (and still does, but that's another essay.) [Hmmph! --Isis] Harry/Draco, however, appeals to many of the same kinks that Snape/Black does – see "enemy slash" above – and I like to refer to it as Snape/Black Lite. But plenty of us still prefer the hard stuff, thank you very much, and as for Sirius – well, for now we'll go with Mostly Dead.
The biggest obstacle
The biggest obstacle to Snape/Black is not their animosity, but the rather unfortunate event that befalls Sirius in OotP. But every fandom has its way of dealing with dead characters, and so do we.
Stories taking place entirely during the boys' schooldays ("MWPP era") are one way of dealing with the problem. Other stories are set during existing canon, although that limits things because there's no canonical evidence of any reconciliation, let alone a relationship. Some stories use Sirius's death as a plot device, and have him brought back somehow - or have Snape die, and the two reconcile in the afterlife. Finally, a few stories simply go AU and ignore Sirius's death entirely. The large number of Snape/Black post-canon stories written before OotP are of course all AU now, which is a bit of a barrier to someone getting into the pairing now.
The initial devices
The antagonism between Snape and Black lends itself to long, plotty stories detailing just how they move from battleground to bedroom. So, how do you get these two guys together?
1. Up against the wall
You'd think that given their animosity, noncon would be common among Snape/Black stories. But you'd be wrong. I suspect it's because noncon generally plays out between unequal partners, and Snape and Black are generally seen as equals, at least when they are adults. But when something tips the balance one of them may snap, as in Fever Dreams (NC17) by Sparrowhawk.
Frequently things begin nonconsensually, but move on to a slow exploration of character, and a happier ending, as in In Cruciatu Veritas (R) by Kleio.
2. I had too much tequila last night
Intoxicating substances - alcohol, drugs, potions, and spells - are a common way of breaking the ice. So Snape and Black get drunk in Oral Sex (R) by Isis, stoned in Under The Influence (NC17, Snape/Black/Lupin) by Fabula Rasa, and are forced to fall in love by a strange magical device in Love Potion HP (PG13, at Thin Line) by Tavayla Ra.
3. Life during wartime
Among the stresses of the conflict between the Order and the Death Eaters, men must find comfort and release where they can. Sometimes one is brought low and the other takes pity (or cruel advantage). Sometimes it's just a quick fuck between equals. Sometimes one needs a favour, or one has simply grown up and wants to atone for his past behaviour. Or it's just one impulse that pushes them across the edge between wanting to fight and wanting to fuck - and it's a bolt from the blue, an irresistible attraction that neither understands but both are helpless to back away from. Stories along these lines include Slowly But Exceeding Fine (NC17) by Ellen Fremedon, Eight Times (NC17) by Fabula Rasa, and Better Call It Home (NC17) by Ailei.
4. I have always loved you
Another common device is to have a schooldays crush lurking in the background. In a MWPP story, this might end badly, leading to the canonical hatred (such as Ulterior Motives (NC17) by Hikaru); in a canon-era story the attraction is revealed as the true source of their animosity (such as in Epistula Ultima (NC17) by Fabula Rasa). Snape has to admit to himself that he has always desired Sirius, or vice versa. Sometimes, they even admit it to each other. Eventually.
5. External pressure
If the men won't get together on their own, perhaps being thrown in a cell together will do it: Velvet Tongue (NC17) by Phantom. Or maybe that old meddler Dumbledore insists they work out their animosity. (It usually ends in sex.) For example, Not Quite Good Enough To Be Going On With (NC17) by Avocado.
The story unfolds
Once Snape and Black are in the same room (and not attempting to kill each other) - then what? To move beyond a PWP, they need to gradually come to terms over time, and this is where this pairing excels. Bringing these men from enemies to fuckbuddies is not such a great stretch; bringing them from enemies to lovers requires the writer to convincingly show their characters maturing and changing through the events of the story. It's a difficult task, but when it's done right it's gripping and emotionally engaging.
The best and most satisfying stories depend on Snape and Black's mutual realization that the other guy isn't the unredeemable arsehole he was previously thought to be, that they really are "two sides of the same coin" - and that they can each supply what the other needs. It makes for a long and delicious plot as they slowly find common ground. Some long, slow fics that do this well are Shade More Than Man (NC17, with R version available) by Acamar, Repechage (NC17) by Fabula Rasa, and Slowly, But Exceeding Fine (NC17) by Ellen Fremedon.
There are not too many established relationship fics in this pairing, for the simple reason that accepting Snape/Black as an established relationship leapfrogs over this process. There are so many barriers to their relationship that most readers need to see it all laid out in front of them. Because of this, the best established relationship stories allude to their previous enmity or flashback to the relationship's development. See for example A Simple Request (PG) and Misericorde (NC17) by Fabula Rasa.
Another subgroup of stories are the Snape/Black/Lupin threesomes. This might be popular because Remus/Sirius is a much more natural pairing to many, as they were boyhood friends, and writers and readers acknowledge it even as they look toward the snark and spark of Snape/Black. It also might just be that Remus Lupin is another interesting and potentially attractive character, as well as their contemporary, and hey, now we're up to three hot guys. Threesome fics include Under The Influence (NC17, Snape/Black/Lupin) by Fabula Rasa, Side Effects (NC17) by Mimine, and Facing The Worst (NC17) by Snapetoy. There is also a master list of SS/SB/RL threesome fics put together by angeline_dark.
It's a wonder to me that Snape/Black is such a relatively uncommon pairing in the fandom, particularly as compared to the incredible popularity of Harry/Draco. After all, both are antagonistic pairings; and in fact, Harry canonically shows more indifference than hate toward Draco, while Snape and Black appear to be pretty well matched in their animosity. (I should add that I wrote this before reading dorrie6's essay here, which discusses this imbalance quite nicely.) Both Snape and Black have greater depth in canon than Draco, and of course they both are adults, making questions of aging-up moot. In fact I think it's the very fact that they are adults that makes this pairing appeal to me: they're not pretty pouting boys, they are, as Fabula Rasa put it in this essay, "the two most fundamentally flawed and damaged characters in Rowling's world – two not-so-good men struggling to be good, and that's the kind of struggle no reader can look away from." The combination of their shared antagonistic history and their separate traumas make them fascinating individually - and irresistible together.
Essays and other meta
A comparison of Snape/Black and Snape/Lupin pairings by cordelia_v
themostepotente asked for advice on writing Snape/Black convincingly
fabularasa mused on Snape's Worst Memory chapter and the character dynamics
smirnoffmule asked about the appeal of Snape/Black - lots of good comments
Snape and Black and Myers-Briggs personality typing by dragonelle_fics
Communities and groups
12am_nosh (for Snape/Sirius and Snape/Regulus)
Thin Line - the Snape/Black archive
Intimate Enemies Snape/Black FQF archive
Snape/Black fics at noire sensus
Snape/Black fics at ISF (needs password - see main ISF page if you don't have it)
Links (not an archive) to SS/SB/RL threesome fics
Nearly all these stories were written before OotP, and thus are AU. Exceptions are Denial and The Last Resort.
A Small Truce by Mariner (R for violence). This is really gen rather than Snape/Black, but it does a lovely job of showing the two coming to terms.
Oral Sex by Isis (R for language). Yep, I'm reccing my own fic here. It's humorous and perhaps not too farfetched and contains no sex.
Denial by Calliope (R). Implied past Sirius/Remus, and it is consistent with OotP.
Repechage by Fabula Rasa (NC17) is a slow and satisfying romance. With a great deal of smut.
Some of my favourites
Shade More Than Man by Acamar (NC17 with R version available)
Slowly, But Exceeding Fine (NC17) by Ellen Fremedon.
Morte O Merce by Cluegirl (NC17)
Fabula Rasa has been called (deservedly) the doyenne of Snape/Black - she's written more words of it than anyone else, and you can find them at her website. Among hers, my favorites (all NC17) are
Rats' Alley, Cordelictus, Misericorde, Repechage, and Eight Times.
Additional classics - not my personal favourites, but frequently recced by others
Side Effects by Mimine (NC17)
Love Potion HP by Tavayla Ra (PG13) (At Thin Line - it's multichapter and chapters aren't linked to each other.)
A Night on the Tiles by Thisveryinstant (NC17)
The Last Resort by Donna Immaculata (NC17)
Resolutions by Josan (NC17)
Isis (includes pairings among Snape, Black, and Lupin)
Painless (scroll down to Snape/Black section)
Titti (the links page has a few, scroll down)