It could be hyperpigmentation. A common symptom of environmental allergies which the other symptoms you mentioned in your first post indicate.
Only further diagnostic testing, preferably by a veterinary dermatologist can answer your questions, as there are several other conditions that could cause this and should be ruled out.
This condition went away after my dog starting allergen specific immunotherapy (desensitization). The discolored skin remains, which is normal. But, no hair loss.
She now tolerates a variety of foods and requires no meds. The ASIT is a natural solution and lifelong treatment.
