My cat has black spots around the mouth on only one side: what is it?

Black spots around a cat’s lips are generally folliculitis from bacterial infection, sometimes spreading to the chin, commonly called black chin. It is usually linked to oily diet, food hygiene, and endocrine imbalance, and also the cat’s constitution; some cats overproduce skin oil, and with drinking motion, black oil builds up around the lips, breeding bacteria and clogging follicles. It is advised to feed natural cat food and clean the lips and chin after meals; for many spots, wipe with saline or a special degreasing shampoo. Also avoid plastic bowls, which breed bacteria and worsen infection.