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Is your dog itchy?
The black on the skin is most commonly from irritation/inflammation. It can be from many different reasons. Most of the time what I see is secondary to allergies with a yeast or bacterial infection. Sometimes they will be missing hair from Cushing's Disease or Hypothyroidism. Sometimes the irritation is from a mite such as Demodex or Scabies.
So your vet will probably do a skin scrape depending on how the skin actually looks...this will look for demodex or scabies mites.
they may do some skin cytology and look at it under the microscope to look for yeast or bacteria on the skin.
thay may want to culture the hair at the edges where it is crusty to see if he might have a fungal infection/ringworm.
Cushing's disease is a problem where the body produces too much natural steroid hormone. Then the dog usually has some other signs too, like drinking and urinating more. Increased appetite. Starting to get a pot-bellied look. So if you notice any of these..be sure to tell your veterinarian. They may want to do some bloodwork or even specialized blood testing.
Hypothyroidism is where the body isn't producing enough thyroid hormones. They will tend to be a little overweight. Not have much energy. Tend to seek warm places even when it isn't very cold. Also can be anemic or have low red blood cells.
The good thing is that for allergies with secondary infections, if you are able to help him stop itching (antihistamines, steroids) and treat the bacteria or yeast, that his hair will come back. Sometimes he will need a medicated shampoo and some type of pill medication.
And he will usually need some rechecks at your vet to get his skin completely cleared up. You don't want to stop treatment too soon or it will come back. One of my favorite medicated shampoos is the Douxo Chlorhexidine. It helps with the skin barrier, has antiinflammatory properties and will help kill bacteria and yeast on the skin. They also have a spray you can use twice a week if you don't want to bathe him that often. With any medicated shampoo you will need to let the shampoo sit on his skin for 10 minutes before rinsing.
If he is pretty healthy then you can try some Benedryl to help his itching until you can get him into your vet. He can have 25 to 50 mg by mouth every 12 hours if he weighs at least 50 pounds. It may make him a little drowsy. But hopefully give him a little relief.