Either black spot disease or healing from ammonia burns or poor water. Goldfish do change colors, but they usually never gain black, only lose it. Black is the most unstable color in goldfish and most black goldfish lose color as they age, but I’ve never heard of a goldfish gaining black, so you know something is wrong.

It’s not Ich. Ich is characterised by WHITE spots on the fish, and it is impossible for them to be black. The white spots you see on a fish with ich is actually a little cyst that the fish’s body produces around the parasite in an attempt to dislodge it.

It is also not “black ich” disease or black spot disease, because that is a MARINE (saltwater) parasite, NOT a freshwater parasite.

It sounds to me like normal coloration. Goldfish change colours over time and it is perfectly normal for them to develop or lose spots or colours.
However, are they actually raised off the fish? Like an actual layer of something? In that case, I might be concerned that it could be a fungus. Do the scales seeem affected or dammaged in any way? In that case, it could be ammonia burn.
However, if it’s just colouration and the other fish are fine, it doesn’t sound like anything to worry about.

Black ich, also called tang disease, affects marine fish and is actually from a type of worm so its not that.

How many fish are in the tank? How long has the tank been set up? Is the black the color of the fishes skin or does it actually look like it is growing on the fish? If it is the color of the skin it could be healing from ammonia burn or something like that and there is nothing you can do to make it heal faster. If it is growing on the fish then it could be a type of fungus or other disease. Either way make sure your water params are good ( zero ammonia, zero nitrite and nitrate around 20).

Mostly likely its Ich a dieases that is fatal and contagious to all other fish, but easily treated with store remedies and medications. It takes a while to completely rid the tank of it, but it generally is treatable and the medications are realitivly cheap. Follow directions and clean tanks as often as possible. Get new filters and really vaccum out the bottom.

It’s probably black ich or black spot disease. While black ich is most common in saltwater tanks, it is a disease that can occur in brackish and freshwater tanks as well. Its symptoms are small black spots present on body and is caused by an organism called a genetic fluke. It is typically seen in newly imported fish and in two forms. Black spot is caught from snails that release the Cercaria or in rocks carrying the fluke. Larval forms penetrate the skin and encyst in the tissue and may be seen as red or black nodules. They lay eggs that block up the blood vessels which in turn causes Necrosis. It is similar to white ich. Please read this link. It can tell you better than I what to do. You can treat it with Ich Attack. However, it could also be ammonia burn. Take a sample of your water to an aquarium to have the ammonia checked. Please see the last link for ammonia burn info.