Allow me to use my knowledge of blues minutiae to dispel a common misconception bred by the previous write-ups. In his original wu, bigmouth_strikes alludes to the truth, but doesn't make it terribly clear. The movie character of Tommy Johnson does not refer to Delta blues guitarist Robert Johnson, but rather to Delta blues guitarist Tommy Johnson.
Tommy preceded Robert on the scene by several years, playing with his brothers primarily in and around the small Mississippi town of Crystal Springs, about 10 or 15 miles north of Robert's birthplace of Hazlehurst. Tommy claimed that he sold his soul to the devil at the crossroads in return for his guitar-playing skills, and went out of his way to exaggerate and perpetuate that story. Tommy was an apt guitarist, but by all accounts unexceptional among the heavies of his time, and as a result his story about the devil was considered by most to be just that - a story.
Robert, on the other hand, underwent a remarkable and drastic transformation in his ability as a blues guitarist in a very short period of time (around two months), during which his whereabouts were unknown to anyone. He resurfaced in the town of Robinsonville where he grew up, at a gig being played by preacher-turned-bluesman Son House. Son used to let Robert play harmonica with him, but always shooed him away from his guitar. "Put that down, boy, you drive people crazy. You can't play nothin'." When Robert turned up and blew Son away with his new musical ability, Son swore that he must have sold his soul to the devil for that talent.
Robert never confirmed the rumors, or even addressed them, which did much more to convince people that it was true than anything he could have said. This, combined with his truly revolutionary guitar playing and the similarity among their names, caused Tommy to be quite eclipsed by Robert. Also, while Tommy may have come along first with that particular story, the crossroads legend was already very well established among southern blacks at that time. Stories of people selling their soul to the devil at the crossroads at midnight for money, love, knowledge or revenge were common among their folk stories for many years prior.