Acoustic Blues guitar buying help

So, I want to learn how to play blues guitar...not the electric stuff, just good ol' natural blues - stuff like John Lee Hooker, Rev. Gary Davis...basically that nice chugging acoustic sound.

Problem is I have no clue what to buy. With my banjo I had to upgrade after a semester of playing...even halfway through the semester I shouldn't have had the banjo I was learning on.

Any recommendations about purchasing a guitar? Budget is around $400.