This happened to a goldfish of mine. I was told that when fish turn black, it means that there is too much ammonia in the water and they're being burned by it. Ammonia to fish is pretty much like acid to us. I think they have kits for stuff like this to test your water for ammonia. It also means you need to clean your tank or bowl more often, because(if I remember correctly) ammonia comes from their poop... so yea. Best of luck with your fish, hope I helped!
its a parasite. its called black ick. takes severe medication and is hard to find. the black is in spots and very hard to cure. no known cause