The short answer
Black on a goldfish has two common explanations. The first is normal colour change — goldfish genetics produce black, bronze and mottled patterns that come and go over a fish’s life. The second is healing after an ammonia burn: black smudges sometimes appear on the skin and fins as damaged tissue recovers, which is a hint that water quality slipped at some point. The way to tell them apart is to test your water and watch the fish’s behaviour.
Normal colour change
Many goldfish shift colour as they mature, and black markings are a normal part of that. Some fish develop black edging on the fins or dark patches on the body that later fade or move. If your goldfish is active, eating and swimming normally and the water tests clean, black colour is almost certainly just genetics doing its thing.
Black as healing from ammonia
Black patches can also be healing tissue after exposure to ammonia. Ammonia burns the skin and fins; as the fish recovers, melanin can gather at the healed areas, showing up as dark smudges. This kind of black usually appears after a period of poor water quality — a stalled cycle, overfeeding, or a missed water change. Once the water is corrected, the marks typically fade over weeks.
What to do
Test for ammonia, nitrite and nitrate. If ammonia is present, do a water change, check your filter and feeding, and confirm the tank is cycled — see what causes an ammonia spike? and how to cycle an aquarium. If the water is clean and the fish is thriving, enjoy the new colour. Keep a test kit on hand and pair it with strong filtration for a messy fish.