Cat World explains that black spots that appear in a cat's mouth are caused by an increase in the number of melanocytes, or pigment producing cells. This harmless condition is commonly called "orange cat lentigo" due to its frequency of occurrence in orange, cream, yellow, and silver-colored cats. Typically occurring before the age of 1, these spots increase in size and frequency as a cat ages.
According to Cat World, these spots can appear as a single lesion or in multiples, and they are characterized as being small, flat or slightly raised black or brown spots with clearly defined borders. When a cat has a single or small amount of these spots, it is called "lentigo simplex," while cats with a great number of spots are said to have "lentigo profusa."
Lentigo is a benign skin condition in cats, but a veterinarian can rule out other skin diseases, such as melanoma, by performing a biopsy, according to Cat World. Using a long needle, a veterinarian takes a tissue sample from the black spot to send to a laboratory for analysis to rule out more serious conditions. While it is unlikely that the spots are serious, it is important to rule out other possible diseases if changes in a cat's mouth occur suddenly.