Hi! I had posted about this before when my pup had a rash between her legs. It was all bumpy and had some whiteheads and it looked even worse because she would lick it all the time, which made it look dark....
Well, when we went to the vet last week for a wellness check before her spaying surgery, the vet looked at it and said "oh, that's puppy acne" its very common and no big deal, he gave her some antibiotics for only 1 week and it was gone in a few days!
Wish I knew this a while ago, when I thought it was allergies and I kept trying to put cream on it. This was a new vet that we switched too, so Im glad he knew what it was.
Not sure if your pup has the same thing, but maybe.....