The black area around a kitten's mouth may be due to the following reasons:
Fungal infection, also known as ringworm in cats. Fungal infections have a relatively long course, generally requiring about a month of medication to improve.
It could be due to food residue or accumulated oil. If the kitten's cat food contains a lot of oil, and its mouth isn't wiped clean after eating, the oil buildup over time can cause the area around its mouth to turn black.
