Here are some of my favorite live blues discs:
Albert King--The Blues at Sunrise (great live set at Montreux capturing Albert's guitar at its most powerful)
Freddie King--the beat 1966 (DVD) (amazing appearances on Dallas TV show; also some later live recordings)
Freddie King--Rockin the Blues Live (Crosscut LP; perhaps never reissued on CD, but the best live concert recording I've heard of Freddie)
Junior Wells and Buddy Guy (Bourbon CD: Japanese, maybe not easy to get but the best live Buddy Guy and Junior Wells release I've heard)
Buddy Guy--Live at the Checkerboard Lounge (JSP CD; not a great night but a pretty good one).
Buddy Guy--the Real Deal (backup band is mostly mediocre, but well recorded and Buddy sounds pretty intense, mostly playing his Chess recordings).
Otis Rush--So Many Roads (the most intense live recordings I've heard of Otis Rush in excellent form).
Since I have been so fortunate as to see all of the foregoing artists live, I might mention a few other blues artists I've seen play brilliantly, though I'm unaware of other live recordings: Fenton Robinson (not sure any of his recordings really captures him, but he sure was fantastic), Melvin Taylor (get any of his Evidence recordings over the last several years), Guitar Shorty (pretty unheralded, but one of the best, still), and Lucky Peterson (hasn't made a complete recording, but a great live blues artist).