I started on Brownie McGhee, whom somebody already mentioned. My father loves Bessie Smith, and I recently got him some Ma Rainey.

We're big fans here of Etta Baker and Elizabeth Cotten, who are more what I think is called Piedmont blues--sort of an early blues/country combination. Whether you like the style or not, Etta in particular was (is?) an amazing guitarist and worth a listen.

I also like Mance Lipscomb but he's very primitive and not as purely blues as a lot of people would like.