I haven't read much Gossip Girl fanfic, to be honest, mostly because it seems limited to Blair/Serena and Chuck/Blair, which I like, but aren't my favorite pairings. My friend
violetion wrote some Blair/Jenny though, and it's absolutely fantastic:
inside out (and all these rules you don't intend to break)- implied Blair/Serena as well, PG-13. Rules can be broken in the subtlest of ways. Highly recommended!