It was an AU where everyone was born with wings and the whiter your wings are the better is your soul.
I think the wings change their color around the age of 12 or something like that.
At this point Blaine's wings become dark brown? and a few years later they become black.
And since black wings are associated with criminals Blaine has a hard time.
and then he meets Kurt (in New York?) who has bright white wings and he doesn't why Kurt wants to spend time with him.
In the end Blaine's wings change their color again and become dark blue. (At least I'm pretty sure it was blue)
Also; I think Cooper appeared in this fic.
I think I've read this fic on ff.net but I'm not sure.
I'm grateful for any help.