I remember reading this fic a while ago, but I don't remember where...

[bananabat16.livejournal.com]

Sebastian starts a rumour that he and Blaine slept together and it results in Kurt breaking up with Blaine and the New Directions being furious with Blaine. They treat him horribly for a few days until two people (pretty sure it was Mike and Quinn) pull him aside and ask him if the rumour is true. Blaine has no idea what they're talking about and when they explain the rumour, Blaine gets upset. They go to Dalton where Blaine gets angry at Sebastian and in anger ends up showing all of the bruises he got from the New Directions' harassment as a result of the rumour. Later when the truth is explained, Kurt tries to apologize to Blaine, but Blaine isn't sure they can make a relationship work if Kurt doesn't trust him and isn't willing to listen.