Max's points say it all, but have a look in "Chasin' that Devil Music" by Gayle Dean Wardlow, Backbeat Books 1998.

He gives a clear conclusion that the story has no basis in fact, how could it? Johnson doesn't make mention of it in his recorded music and although some of his contemporaries made reference to the idea they later denighed it really meant anything.

Johnson was an exceptionally talented man and we are exceptionally lucky to be able to hear him and his music