Goldfish tend to change colors as they grow. Once they are around 6 inches in length, the color will become stable and they will remain that color the rest of their lives.

It's a good sign that your fish is still eating and swimming around, which means it's probably healthy.

I would also do a partial water change around 20% just in case the black color is a result of ammononia burns, which often is a result of dirty water from overfeeding and not doing water changes regularly. This will substantially lower the ammonia level in the water. Remember goldfish produce a lot of wastes so water changes no more than 25% once a week is a must.

The black bits could be amonia burn!!! But as Izzy said it might be rough handling. Plz post a pic so we can see 4 ourselves!