The "black man" is the Devil, although in the context of this story he goes by the name of Old Scratch, or the Black Woodsman. He isn't black in terms of African ancestry, but in terms that he is literally black, suggesting a supernatural origin. The specific reason given for Scratch being found in the swamp is that he had always lived there, long before the land was inhabited by the current settlers.
Scratch doesn't really try to "do" anything to Tom, or with him, at least not at first. He acts somewhat protective and dismissive and doesn't really invite conversation with Tom. This may be because he knew the sort of person that Tom was, and the suave and smooth-talking nature that the Devil is normally depicted with was unnecessary to convince Tom to go along with his plans.
Eventually Scratch lets on that he knows the location of a buried treasure nearby, and he will give Tom this treasure in exchange for certain "conditions". These conditions are not explicitly stated but it is implied that Tom is trading his soul for the treasure.