The only time I've heard of this practice was in the movie "Crossroads" w/ Ralph Machio having to beat Steve Vie, ( the Devil's guitarist), in a duel for his soul. It was the best scene in the movie because Steve Vie was actually playing his parts. Of course Machio wins by tripping Vie up w/ a complicated classical piece.
I don't know if this scene is based on an actual practice among Blues guitarist or not, but it made for good cinema.
Here is a clip:
http://www.youtube.com/watch?v=djIXHfh5GcE