Goldfish often turn black due to genetics, according to FishChannel.com. A goldfish can change color several times over the course of its life. When a goldfish begins turning black, it is often not a serious issue and is most likely due to natural causes.
Another reason goldfish turn black is due to their environment. Goldfish can tolerate a variety of tank conditions. However, according to PawNation, too much ammonia can cause black chemical burns on goldfish. Too much ammonia is frequently caused by uneaten food, waste and plant debris.
Goldfish can also turn black due to disease, according to The Nest. Water snails can carry a disease called black spot. Eggs of parasites found on water snails burrow into the skin of goldfish and cause black patches. Parasite infestation can be remedied by removing water snails from the fish tank.