But thanks to segregation, most white Americans still knew little about the blues before the mid-1960s. Ironically, it took a wave of British acts to bring the blues "home" to the white mainstream. Bands like the Rolling Stones, the Animals, The Yardbirds, and Eric Clapton's Cream took songs by Howlin' Wolf, Jimmy Reed, and other blues originals, turned up the volume, and rocked the blues. Suburban white kids filled nightclubs and stadiums to hear the Brit bands and home-grown blues stars like Janis Joplin. While most of the riches still flow to rock-and-roll and country artists, the later careers of Stevie Ray Vaughan, The Fabulous Thunderbirds, and Bonnie Raitt prove that, packaged the right way, the blues can speak to a white audience. Which begs the question: What does an audience, black, white, or other, hear in the blues? What's the source of the power that creates such fervent loyalties?

"The core of the blues is emotional honesty," says Peter Guralnick, author of Searching for Robert Johnson and a definitive two-volume biography of Elvis Presley, Last Train to Memphis and Careless Love. "It reflects honestly and directly the emotional condition of the moment. That may be celebration, it may be utter dejection, but it gives you what the person is feeling."

Seconding that notion, producer Alex Gibney says the blues' emotional intensity makes it a needed counterpoint to much of the dance-machine fluff crowding the airwaves.

"In an era when so much music is manufactured, the blues provides an important tonic because it feels so true, so raw, so immediate," Gibney says. "It's not always PC, and it often connects with the way we are rather than the way we imagine ourselves to be. But you gotta sing about that, you gotta get that. Otherwise, as Freud once said, the repressed always comes back to haunt the repressor."