Goldfish changing colour

The black is caused by healing from ammonia burns as well. A fish which has
had an injury or other condition, like ammonia burn, will turn black in that
area as it heals, and it will return to normal in a week or 2.

I do agree that you should test the water since 8 fish should be in a rather
large tank (at least 10g per adult fish, adjust for size/age). I would
think your fish is healing from a previous condition so you are doing
something right. Keep up the partial water changes and keep an eye on the
water by using test kits - ammonia, nitrite, nitrAte.