How old is the dog? This could just be hyperpigmentation that often comes with age and sun exposure. Could be adrenal or thyroid issues. Does the dog have any other symptoms at all regarding the skin, hair loss, etc?
I assume you asked the vet about this? If you are that concerned, you should go back and speak to your Vet about this.
I'm going w/ "Yeast Infection".....Please, look up "Nzymes.com" & read everything. I believe you'll find your answer there......Take care, Karla