A black circle around the edge of a cat's lips may be caused by a variety of reasons, such as food residue, folliculitis, trauma, or cat ringworm. - Food residue: If the cat does not clean its mouth after eating, food residue may remain in the corners of the mouth. Over time, these residues may turn black due to oxidation. - Folliculitis: Folliculitis is an inflammation caused by the overproduction of bacteria in the hair follicles, which may cause symptoms such as redness, swelling, heat and pain, hair loss, and small pustules on the cat's skin.

- Trauma: If the cat's local skin is hit or squeezed, it may cause skin damage, which may lead to bleeding, redness, swelling and Symptoms such as pain, and over time, the blood may oxidize and turn dark.

- Cat ringworm: Cat ringworm is a fungal infectious disease that may cause the skin to fall off. In severe cases, it can cause damage to the skin, causing melanocytes to precipitate and darken the skin.