A fish's eyesight is linked to its chromophores which give it color. Blind or partially blind fish are usually very dark almost pitch black.
Thanks Cecil, I've never heard of this before but you've described him perfectly. He is the only "black gill" in my pond and now I find out he is handicapped. But thats cool though because he is now off limits for anyone fishing my pond. As you know, I'm a sucker for stuff like this but I love it.
I'm going to look up "chromophores" next.