The story I'm trying to find is an "Blame It On The Alcohol" AU. While at Rachel's party, Puck's in the bathroom when Blaine comes in drunk. For some reason I can't remember, Blaine needed Puck's help with getting his pants down, to which Puck comes face-to-face with Blaine's naked ass, and is mesmerized by it, calling it a getto-booty. They end up having sex. Afterwards, Blaine becomes addicted to Puck's cock, and Puck can't resist Blaine's ass. I think this story was on fanfiction(dot)net, but I can't be sure.