Robert “Mack” McCormick’s life was all about the blues. His Houston home was stuffed with reel-to-reel tapes, research files and vinyl records. But by the time he had died at age 85 in 2015, few really knew what his collection contained. That included his daughter, Susannah Nix. She was 3 when he finished his last significant project, the liner notes for a 1974 album featuring 1920s recordings by Texas musician Henry Thomas.