Weird Black Scab Under Chin?

[muffinmclay]

I noticed tonight that my dog has a black crusty scab under his chin, about the size of a 5 cent piece. He pulls away when it is touched, so i imagine it is sore. Any ideas as to what this might be? We have two dogs and they often bite each other around the ears & muzzle when playing, maybe it could be a bite and the black is dried blood? I have no idea, but would like to avoid an unneccesary vet visit if possible.