Black scum on a cat’s tail may be caused by parasitic infection or folliculitis. First, check whether there are parasites on the cat. If there are parasites, the cat needs to be dewormed externally in time, or you can directly take a medicated bath. ; If there are no parasites, you need to pay attention to the cat's diet and hygiene. Do not feed the cat food that is too oily to avoid excessive oil secretion.