Let me mention the late great Johnny Adams, an absolutely phenomenal R&B/jazz singer not widely known outside New Orleans and my all-time favorite male singer. Maybe somebody's curiosity will be sufficiently piqued to look him up, maybe even find some free sound clips.

A few more that pop into my head at the moment:

Tony Bennett

Otis Redding

Mississippi John Hurt

Janis Joplin

Joe Cocker

Henry Roeland "Professor Longhair" Byrd

I agree with all the above choices that I know; a couple of the names are not familiar to me. I suppose I should follow my own advice and look 'em up.