-Blaine was pregnant,he told to Kurt but Kurt ran away because he thougt it was somebody else's baby.(I don't know, maybe Eli?)
-Kurt was refusing to talk with Blaine. Finn knew it was Kurt's,he was feeling pity for Blaine.
-When Finn and Kurt looking for baby clothes ( for a baby shower if I remember correctly), they bumped into Blaine in a baby store.
-Finn was seing how devastated Blaine was,one day he couldn't take it anymore so he exploded with rage to Kurt.When Finn said Kurt is the father,he apologize from Blaine.
I probably read at fanfiction.net or scarvesandcoffee.net but it can be also AO3.
Thank you for your help!