The documentary filmmaker Les Blank, who died in 2013 at the age of 77, did the lord’s work. Likely best known for “Garlic is as Good as Ten Mothers” (1980), “Burden of Dreams” (1982), and “Gap Toothed Women” (1987), Blank’s primary enterprise was the chronicling of American vernacular music. From Dizzy Gillespie and Lightnin’ Hopkins to Clifton Chenier and the Wild Tchoupitoulas, Blank set out to valorize cultural byways that were off the beaten commercial path. In doing so, he preserved many of them for the historical record.