There are times when black spots on the skin are a normal occurrence, states Petfinder.com. These spots can usually be found in areas with constant friction, such as under the legs. However, black lesions on the skin can be a symptom of allergies. It is only a problem if the dog is bothered by the area in question.
Petfinder.com also states that black spots are normal for aging dogs. Fleas are not a culprit in black spotting, since parasites usually cause hair loss on the animal. A reaction from major allergies can develop, most likely stemming from a problem called lichenification, in which black spots are itchy and rough in appearance, similar to elephant skin.
According to WebMD.com, dogs can also get what is known as fly-bite dermatitis, where the dog is bitten around the floppy portion of the ears, resulting in black scabbing from bleeding. Red spots are also an indicator of insect bites.
EarthClinic.com notes that if there is no physical disruption in the under-surface of the skin, then it is nothing to worry about. However, if the dog exhibits irritation and hair loss, special creams or ointments can be applied, or a trip to a veterinarian is in order.