Ammonia burns are the most common reason that goldfish turn black. High ammonia content in fish tanks causes irritation and trauma to the goldfish's fins. Ammonia burns are white in color and turn black as the goldfish heals.
Additional reasons for a goldfish to turn black include ingestion of colored fish flakes and genetics. Many goldfish possess a random mix of color genes, which in turn can lead to color variations.
High ammonia levels in a fish tank are easily remedied with a 25 to 50 percent water change. Ammonia levels tend to fluctuate when there is undigested food or fish droppings in the water as these items emit excess ammonia while decomposing.