Black Spots On Skin

my bull terrier Staffordshire suddenly my skin is black under the arm before (armpit). there are black spots that differ in size, which are mainly under the arms before, but there is a little on his chest and down the little front arms?

The question does not specify how old the dog is or its history.
Always safer to have a diagnosis of problems of VET.
I saw mill dogs (women) with a pigment dots under the arms, belly on down.That proved to be a hormone imbalance of being raised near death.
Maybe he sleeps on (cement?) A skin disease or any number of things.Flea feces, everything is speculation
In case of doubt on foot to the vet ...

Is it fur? Some dogs black fur over time. If not fur, I think you should go to a vet. Make sure it is not fur first. Good luck!

Your dog probably has an infection of the skin ... my mother is a veterinarian ... If you're in San Diego, I can send to his free ... my dog had that too ... sorry, but it is difficult to deal with

If they are unusual for your dog, then take him to the vet and make sure you tell the vet they were not their before.

flesh-eating virus?