In the mid to late 70s I lived in the Hollywood Hills and hung out with a number of musicians, Carlos Guitarlos and Top Jimmy (of the Rythm Pigs) were among them, after the bars closed, we would head into the Hills, get stoned and keep playing music, at some point during the night, it would always be time for everyone to put down their guitars, and listen to that old Robert Johnson record once again.
Did he use a pick or just his fingers... either way WOW WHAT A TALENT!!! I think he was dedicated to his craft and worked hard to become the legend he is. He had a tough childhood and lost both of his wives so it only seems natural he would sing about the devil and hardship. RIP