Wow, some good stuff in this thread. Well worth the necro Nitro_X. I'll add a few to the list (some from far out in the left field)...
Bonnie Raitt, (Traditional Blues/Country) probably the best female blues artist ever. Had the immense pleasure of experiencing her incredibly moving live performances. Hard to pin down one song as a sample.
Frank Zappa (yes he wrote some blues numbers and covered others) always had amazingly talented artists with him.
Jimmy LaFave (Leaning more towards folk/country ballads) an accomplished acoustic guitar player with a sweet voice. Some beautiful Dylan covers.
Leonard Cohen Way beyond blues and deep into Black (so dark it could almost be satirical). Cohen's music is not advised if you're prone to suicidal thoughts.