Have you checked your water parameters for ammonia, nitrite, pH and nitrates?
If not please test your water and post the test results here!
What size tank do you have?
How many fish in the tank?
What kind are they and how big?
When black just shows up on a fish this is not good and could be possible burns from ammonia present in your tank.
Do you have a filter in your tank, if so, what kind is it?
Do you use a water condtioner?