BIID is a disorder that causes someone to want to be disabled. It most often causes people to request that perfectly healthy limbs be amputated and sometimes when doctors refuse, patients have been known to injure themselves in such a way that amputation becomes necessary. It can also manifest in a desire to be blind or deaf.

So, Foggy has wanted to be blind for as long as he can remember. Most likely he has never told anyone about it for fear of what they would think, but he devotes a lot of his time to research, he blindfolds himself sometimes and he maybe has learned to read Braille a bit or something like that. Then he gets to college and meets Matt and it is absolutely the best thing that has ever happened to him. Only he can't say anything about it to him because he doesn't know how he'll react.

Writers choice from there, maybe meeting Matt helps to change his mind, but most likely becoming friends with such a capable blind person only spurs him on. Maybe he tells Matt and Matt reacts one way or another, maybe Matt finds out on his own becuase he finds something of Foggy's that doesn't make sense (Braille book or something?) and asks him about it..