Even if the market-driven “diversity” of the Beale Street Music Festival lineup paradoxically results in a somewhat predictable slate of artists, surprises can be found, sometimes in unlikely places. Saturday afternoon, at the so-called SoCo [i.e., Southern Comfort] Blues Shack (a small stage added to the festival this year to give area blues artists some Memphis in May exposure), Bluff City-based hill country stylist Richard Johnston followed a typically rousing North Mississippi shake-’em-on-down foot-stomper with a cover he said was inspired by a soldier friend’s recent return from Fallujah in Iraq. “I’m gonna dedicate this song to the Commander in Chief,” Johnston said; then, he and his slide-guitar sideman launched into what I eventually happily recognized as a blistering blues rendition of Black Sabbath’s Vietnam-era “War Pigs.”

Such political consciousness may make Johnston a more “authentic” bluesman than some of the tradition-bound performers in the much larger Tennessee Lottery Blues Tent; don’t forget (to name only two examples), John Lee Hooker recorded “The Motor City Is Burning,” about the 1967 Detroit race riot, and J.B. Lenoir’s discography includes “Eisenhower Blues” and “Vietnam Blues.”

Will Johnston repeat his Ozzy tribute/Dubya diss? Find out today [Sunday]: He’s scheduled to play at 2, 4:40, 6:20 and 8 p.m. at the Blues Shack.