BKblues,
Try the Full Moon Saloon, 1710 Aliceanna St. in the Fells Point section of B'more. They specialize in blues (and booze), usually no cover. You might also contact the Baltimore Blues Society at 410-329-5825.
Both places are heavily electric blues oriented, but occasionally acoustic acts come by. Some of the work is the same, electric or unplugged.
There is also a new restaurant on the Avenue in White Marsh that is supposed to feature blues performers. Have not been there myself.
Good luck and enjoy the music!
Roger in Baltimore