Spoilers: Through end of Sentinel, minor spoilage for SG-1 1st season and Meridian
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Personal Website: The Gardendoor Fan Fiction Archive
While many writers attempt crossovers, some more successfully than others, it's unusual to find a pairing that is both highly popular and easy to write. Blair Sandburg and Daniel Jackson, though, fit the bill perfectly. When I first read this pairing, I'd actually never seen either show, and had never even met the SG-1 crew in fanfic, but I was struck by how the two characters complemented each other, how they comforted each other, helped each other gain perspective and find their way past seemingly impossible, life-changing obstacles: the loss of a wife, the death of a career, years of unrequited love for a seemingly indifferent partner.
Blair and Daniel are both crackpot theorists whose theories were unexpectedly proven right, ripping them from quests for dry, dusty footnotes to the shock of life-or-death reality. They each find themselves risking their necks and even dying for their best friends, gruff military-types who jokingly disparage them as geeks. They're both extraordinarily bright and energetic, throw themselves wholeheartedly into their work, and go toe-to-toe with people when their principles are stepped on. And both are spiritual seekers who keep an open mind about the supernatural. Even their quirky backgrounds lend themselves easily to connection, always the hardest sell in any crossover: Daniel Jackson was orphaned at an early age, Blair Sandburg's flaky single mother could easily have left him with friends or lost custody of him temporarily, making it plausible for the two boys to have grown up together. But as they've both studied in obscure libraries and traveled extensively to exotic backwaters, they could meet up any number of ways.
Despite the many similarities, though, Blair and Daniel aren't clones. They aren't even equals. While Daniel gets ribbed for not being military and being a bit of a geek, he's a PhD who knows dozens of languages and cultures. He's one of the two amazing brains of the team, and is respected for his unique strengths by his teammates. Despite his academic background, he's never objected to carrying a gun as part of SG-1, or even using said gun. He's been described as the conscience of SG-1, and he does fight for his convictions, but he's a cultural relativist, rarely objecting to offensive customs within another culture, only within his own. For example, while he's completely against the Goa'uld or anyone else enslaving or wiping out a culture, when facing cultures that seem to believe in marital rape (Broca Divide) or the death penalty (Cor-Ai), he seems content to just observe and analyze unless goaded by his friends into action. But the real kicker of inequities is that while Blair is the one who meditates, the one who has been charged with the responsibility of shaman, Daniel is the one who has ascended to a higher plane and learned the secrets of the universe. The only time Blair even had a vision was when he was piggy-backing on Jim's.
Blair was a child prodigy but tapered off, and is still struggling to get his doctorate at age thirty despite starting college at 16. He has to fight to be heard by Jim and the Major Crimes department, even when he's proven right over and over. He hates guns, preferring to use his wits or handy pieces of furniture to defeat any criminals stalking him. He's hyper where Daniel is mellow, the eager kid brother. And despite his many years as an anthropologist, he's anything but relative when it comes to ethics; he'll jump right in there if he feels someone's being hurt or taken advantage of. Usually when he's paired with Daniel he's relieved by the opportunity to be treated with respect and gentleness by someone who shares his interests. But Blair has something Daniel does not: He knows, because of the Sentinel-Guide link, that he is irreplaceable to Jim, no matter what frictions divide them. Jim will always need his Guide. Daniel has no such assurances from Jack, and is also stuck in the "Don't ask, don't tell" trap that could cost both him and Jack their careers as members of SG-1 if they even raised the possibility of a relationship.
The advantage of a similar-but-not-the-same pairing is that Blair and Daniel each know where the other is coming from, and they can see in each other's lives a glimpse of how their own lives could be different. They can goad each other into changing course or pulling themselves out of a rut because they're alike enough to know exactly which arguments will best persuade. But because of their work commitments and their wanderlust natures, this pairing lends itself more easily to on-again, off-again romance or wacky, one-time comedic disasters, not pledging undying love and setting up housekeeping. Dangermouse was my first introduction to the joy of Daniel/Blair, and Caro Dee's Dog Series is also really good. Learning Experience was a cute glimpse into an early B/D ship, and I've always liked the way Tryst shows how they can help each other and find common ground even when they aren't the primary pairing.