Dogs can either be born with dark spots on their hides or they may suddenly form. There are several different reasons why the dark spots appear. While there are times when the dark spots might indicate a health problem that should be treated, other times the spots don't signify anything other than a change in the appearance of the dog's skin.
Primary Acanthosis Nigricans
This rare disease affects puppies that are less than 1 year in age, and it usually only affects the Dachshund breeds. If your dog is affected by this condition, its skin will thicken and darken and affect large portions of the dog's body. While this particular condition is not curable, it is treatable with steroid melatonin injections and special shampoos. This condition can lead to secondary infections such as bacterial infections.
Secondary Acanthosis Nigricans
This condition is more common than primary acanthosis nigricans and can affect any breed of dog at any age. This condition is usually caused by friction due to a dog's obesity, hypersensitivity to food or hormone imbalances. In order to treat secondary acanthosis nigricans, it is best to treat the underlying causes such as a healthier diet to fix the obesity problem. The black spots will recede once the underlying causes are addressed.
It is not unheard of or even uncommon for black spots to show up on a dog's belly as it ages. This does not hint at any sort of health issue as long as the black spots are not scaly, do not have an odour or are not accompanied by other disorders such as hair loss.
If the black spots appear tender to the touch or are changing in size and darkness, it could be an indication that your dog has a thyroid condition. Generally, the dog will also lose some hair as another symptom of this condition.