I am sorry to hear you are worried about Sable. I am a veterinarian and will do my best to help.
Does the dark gray skin have normal hair, or is the hair thin or missing? Sometimes dogs with chronic allergies will get a secondary yeast infection from all the scratching. Yeast is very itchy, so then they scratch even more. Usually if the skin turns gray from a yeast infection, there is hair loss, and the skin is also thickened and moist. If you could post a picture of her skin, that might help.
If this is due to a yeast infection, there are two parts to treating. One, we treat with anti yeast products: oral medication like fluconazole; topical yeast medication like miconazole; and bathing in shampoos that contain miconazole or ketoconazole. Some of these are available over the counter, but you need a prescription for the oral medication.
Two, control the itching. If he keeps scratching, the yeast infection will not go away. This can mean prescription medications like prednisone, over the counter anti histamines like Claritin or Benadryl, or the newest allergy medication for dogs called Apoquel. About 80% of dogs improve and have their itching controlled with Apoquel. You would have to get this from your veterinarian.
There are also hormonal disorders like hypothyroidism and Cushing's disease that can cause these kind of skin problems. We do blood tests to diagnose these, and treat with prescription medication.
Let me know if you can post a photo, or have any more questions. Rebecca