When director Robert Frank took off in his car from Birmingham, Ala., for location shooting in Nova Scotia, he had with him what was thought to be the only existing print of his film “C . . . Blues,” a controversial documentary (with an obscenity in its title) about the Rolling Stones’ 1972 American tour. The film was scheduled for a midnight screening last Saturday at the American Cinematheque’s “Rock on Film” festival at the Wiltern Theatre. Festival organizers frantically phoned rock radio stations across the country, asking them to page the airplane-shy director and get the print back to them. Frank has still not been located, but festival co-sponsor Bill Graham discovered that Mick Jagger had the only other print, now on its way from London. A screening tentatively has been rescheduled for Jan. 16.