Watch the Judgement Day online guitar lesson by Jeff McErlain from Blues Guitar Fakebook: Rhythm Vol. 2
Here's the classic riff Eric Clapton uses on his version of Robert Johnson's "If I Had Possession Over Judgement Day". This groove has been used on many tunes such as "Rolling and Tumbling", "Stones in My Passway", and many more. I highly suggest listening to Robert Johnson, I find a lot of people know his name but not his music. I also suggest Eric Clapton's Me And Mr. Johnson record for a modern take on the Johnson catalog.