It may be folliculitis if the corners of a cat's mouth turn black and appear. This is usually caused by improper diet, unclean cat bowls, or the cat not cleaning the corners of its mouth in time after eating. You can use sulfur soap or cat shower gel to treat it. Help the cat remove black scabs, and at the same time improve the cat's diet and feed it some lighter food. It is best to clean the cat's bowl every day to avoid the breeding of bacteria. If the cat's mouth is dirty after eating, it should be wiped clean in time.