In the sprawling, ever-evolving landscape of rock music, few bands occupy a space as singular and enigmatic as Procol Harum. They emerged from the psychedelic chrysalis of 1967 not with a fuzzed-out guitar riff or a hippie-dippy singalong, but with the stately, mournful chords of a Johann Sebastian Bach cantata. With the release of “A Whiter Shade of Pale,” they didn’t just score a hit; they invented a subgenre: Baroque 'n' Roll.