Watch the Raising Sand online guitar lesson by Jeff McErlain from Blues Guitar Fakebook: Rhythm Vol. 2
As an aside, I want to list some blues musicians you should really know about and give a listen. Some of the old Delta stuff is hard to listen to due to the poor audio quality, but there is some great music in there. Charley Patton, Skip James, Son House, Tommy Johnson, Memphis Minnie, Leadbelly, Blind Willie Johnson, Blind Lemon Jefferson, Bukka White, and Lonnie Johnson to name a few.