Rachel's short lived relationship with Blaine began at a party in which Blaine kissed Rachel in a game of spin-the-bottle. Rachel, pleased with this potential duet partner, asks Blaine on a date. Blaine agrees to the date and questions his sexuality, thinking he may have enjoyed the kiss shared with Rachel.
Blaine kisses Rachel yet again - this time sober, however - and reaffirms his confidence in his sexuality being entirely homosexual.
The two are still friends.
After foolishly realizing he led Kurt Hummel on, another homosexual male who loves Blaine, Blaine explains that he's no good with relationships. The two manage to handle themselves just fine, however, as they are hastily kissing and become boyfriends.
The two's love doesn't end when Kurt leaves the Warblers, either. He tells Kurt that they will still be together and is answered, "I won't leave you, ever". Blaine Anderson later joins William McKinley High School to be with Kurt Hummel in Season Three. The two grow increasingly close until The Break-Up in Season Four when Blaine cheats on Kurt while Kurt is in New York.
The two slowly mend their relationship after "The Break Up" and when Blaine eventually expresses desire to attend NYADA after graduation, chemistry clearly reestablishes itself between the two. In I Do, the two become "friends with benefits," as their relationship slowly turns back into the love they once shared.
In Wonder-ful, Blaine asks for Kurt's dad's word of approval if Blaine were to propose to Kurt. Burt, Kurt's father, acknowledges the love the two of them share but asks Blaine to hold off on making such a life-altering move at such a young age. Blaine's disappointment in this response doesn't change his mind, however, and he continues the plan to propose.