The reason I had you check his gums, and for bruising, is because they can have blood clotting disorders which cause bruises appear on the skin and gums.
So, there is no bruising.
Then, it sounds like allergies. The arm pits, and groin, are where they sweat the most. So if they have allergies, this is where they are most affected. You can try applying, over the counter, hydrocortisone cream, or spray, three times a day and see if you can reduce the inflammation.
In addition, you could try giving him an antihistamine such as benadryl. The dose for benadryl is 1 mg per pound every 12 hours. For example, a 10 pound dog would get 10 mg of benadryl every 12 hours. Use the human kind you buy at the drug store or grocery store.
If the skin is black, that indicates chronic inflammation.
Once you get rid of the cause of the inflammation, then the color should return to normal. It will probably take at least a week to see a color change.
If the skin is red, and hot to the touch, that would be an infection and require antibiotics for treatment.