For early country blues (maybe even 'pre-blues') I recommend Papa Charlie Jackson of New Orleans. He generally recorded solo banjo and vocals, and he recorded about 60 sides in the 1920s and 1930s. His repertoire included stuff that most listeners would identify as "blues" as well as ragtime-sounding and vaudeville-influenced stuff. I should mention that there is a rumour he taught Big Bill Broonzy how to play guitar, but I don't know if there's any truth in that.

The stuff I have is on LP on the Biograph label, but I'm sure his stuff has been re-released on CD.