“Chulas Fronteras,” Les Blank’s humane and informative 1976 documentary on the music of the Texan-Mexican border, screens at the Roxie Theater on Saturday, Aug. 26, as part of the movie house’s series “Frontera!,” films that deal with the U.S.-Mexican border. To a rich soundtrack of songs about unrequited loves and migrant struggles, Blank interweaves the stories of all the people involved with musica norteña: recording artists, radio DJs, consumers and, one of its perennial subjects, migrant workers. Blank was one of America’s keenest documentarians of regional musical traditions — blues, Cajun, Appalachian, Creole, Hawaiian and more.