Charlie Musselwhite has a terrible confession to make: although he and Ben Harper are currently covering “When the Levee Breaks” in their concert sets, he’s never heard the famous sludge-metal take on the song that Led Zeppelin cut in 1971.

“Actually, I’ve only ever heard the Memphis Minnie version,” the veteran harmonica player reveals with a laugh, in a telephone interview from his Sonoma County, California, home. “I’m a little out of it. I have practically no knowledge of modern music after about 1953.”

In Musselwhite’s case, ignorance might just be bliss. And Harper is using the Mississippi-born 74-year-old’s authentic blues harmonica sound to good advantage, both on the road and in the studio. Their new record No Mercy in This Land—a follow-up to 2013’s Grammy Award–winning Get Up!—is one of the younger musician’s finest, and a great showcase for Musselwhite’s blend of gut-punch immediacy and tonal nuance. But then, that’s not surprising when their union was initially blessed by none other than the high priest of boogie, John Lee Hooker.

“Ben and I first met in 1993,” Musselwhite explains. “John Lee Hooker was playing a little club in Mill Valley, California, called the Sweetwater, and Ben was opening for him. I was just sitting in with John; if I wasn’t working on the road myself, often John would call me and ask me to come down and play with him.

“He never paid me anything, but I’d go anyhow,” he adds, laughing again. “It was always fun. So it wasn’t long after that first time we met that John Lee had us, just me and Ben and his rhythm section, come in and back him up on one track for an album [The Best of Friends]. The tune was called ‘Burning Hell’. We really locked in in the studio, and even John Lee said, ‘You guys ought to record more together,’ ’cause we sounded so good.

“So we stayed in touch and kept talking, but we were both so busy we never had time to, like, sit down and do a whole album,” Musselwhite continues. “But finally, after I don’t know how long—a decade, a least—we had the time at the same time to go into the studio. And it was like the music had been waiting all this time. It just came charging out like wild horses.”

That’s evident on both Get Up! and No Mercy in This Land, but it’s especially true on the second effort. Sounding at times like an effective and intuitive updating of the Chicago blues sound, No Mercy in This Land offers Musselwhite lots of room to show off his skills; at times, his harmonica sounds like a second voice, offering emotional commentary on Harper’s distinc­tive vocal combination of restraint and intensity. (Harper’s slide guitar serves the same function on his solo albums, but here gets only a single extended showcase, on “The Bottle Wins Again”.) And while it’s entertaining to think of No Mercy in This Land as a role-reversed take on the “fathers and sons” albums of the 1960s, with an old white bluesman tutoring a younger black apprentice, Musselwhite stresses that Harper has ample blues cred of his own.

“He grew up in a music store—like a folk-music store, not a Top 40 music kind of place—where they sold instruments and records and books, so he’s well-versed in all kinds of folk music, including blues, from around the world,” he explains. “And I’m interested in that.…When I was growing up in Memphis, going around looking for old 78-rpm blues records, anything else that looked interesting, I’d buy that too. They were only a nickel or a dime back then, so I discovered a lot of music—like Greek rembetiko music, which has a bluesy sound to it, or flamenco, which has that toughness. That’s what led me to thinking that every culture maybe has its own kind of blues.”

Singing the blues, he adds, is one of the best ways of exorcising pain—a notion borne out by No Mercy in This Land’s title track. Written by Harper but sung by Musselwhite, the song was inspired in part by the 2005 murder of the harmonica master’s elderly mother.

“I couldn’t write a song about that situation; it’s too personal,” Musselwhite says. “But Ben has a perspective, some distance on it, so he could do that. Every night, we do that tune, and it’s just a great feeling. It makes me feel… I don’t know if good is the right word, but it’s a balm.

“That’s what blues has always been about: talking about the truth and, you know, reality,” he continues. “If I were in country music, I’d sing ‘My baby left me, and I want to go jump off a bridge.’ But in blues, the guys sing ‘My baby left me, and I’m gonna go get a new baby.’ The spirit of the blues is ‘We can get through this.’ ”

Charlie Musselwhite and Ben Harper play the Orpheum next Thursday (August 23).