How old is your puppy? When Snickers was a puppy, up till he was 5 months old he had black dirt like stuff around his privates. At first I thought is was flea dirt but if we had fleas our Boxer (who sometimes I swear is allergic to himself) would be covered in hives. At one of snickers puppy visit I asked the vet to check it out. She told me that is was just puppy acne, just like human babies get and that it should resolve on its own, and it did.
Maybe, that is the case with your pup.