The blackening of the inner wall of a dog’s mouth is due to pigmentation, which is a normal phenomenon. Some dogs are born with dark spots in their mouths, and some dogs are caused by pigmentation due to age. As long as the dog’s mouth does not appear, There is no need to worry about other abnormal situations. If your dog suddenly develops blackening of the lining of its mouth, it is best to take your dog to a pet hospital for examination.