ON FEBRUARY 21, 2012, at the end of a White House blues night, President Obama sang a chorus of Robert Johnson’s “Sweet Home Chicago.” Two weeks later, on March 6, to mark Johnson’s centennial, the Apollo Theater in New York staged the tribute concert “Robert Johnson at 100,” featuring, among many others, the Roots, offering “Milkcow’s Calf Blues,” Living Colour with “Preachin’ Blues,” Elvis Costello with “From Four Until Late,” both James Blood Ulmer and Taj Mahal taking on “Hellhound on My Trail,” Bettye LaVette with “I’m a Steady Rollin’ Man” and “When You Got a Good Friend,” Shemekia Copeland