If this is not normal than you should see a vet to ensure that it is not some form of skin cancer or other skin infection that can spread and cause more problems.
Did you check that they are not ticks or fleas? if you can remove one and if it appears to be a tick or a flea, then there are many products that you can buy to treat your dog.
This is nothing to worry about. Dogs commonly get black spots on their underside. It is the human equivalent of freckles. They usually show about after the 1st year of the dogs life. Its harmless.
