Gold fish are dirty fish and need constant water changes. At least 10% weekly. 1 goldfish needs about 20 gals of water, a second gold fish would need and additional 10 gals. Small tank will need more water changes then a larger tank. Usually the black is an ammonia burn caused by dirty water ( needs 50% changed). I have a fancy tail goldfish in a 50 gal
tank and do a 10% change weekly. If I miss a week or 2 the goldfish gets stressed and sits at the bottom of the tank. I do a 10% change and the next day its swimming around happily. Good Luck! I know the pain.