At the beginning of Pamela B. Green’s lively and informative new documentary, a bevy of movie people — directors, actors, scholars and others — are asked if they know anything about Alice Guy Blaché, who is the subject of the film. A few of them do (Ava DuVernay, for one), but most admit that they have no clue. Viewers who are similarly ignorant shouldn’t feel bad, and in the best pedagogical spirit Green turns blank looks and sheepishly shrugged shoulders into a teaching moment. “Be Natural: The Untold Story of Alice Guy-Blaché” seeks both to help rescue Guy Blaché from oblivion and to explain how she got there in the first place.