“The Black Power Mixtape 1967-1975,” among other things an extraordinary feat of editing and archival research, takes up a familiar period in American history from a fresh and fascinating angle. In the late 1960s and early ’70s, Swedish television journalists traveled to the United States with the intention of “showing the country as it really is.” Some of the images and interviews they collected have been assembled by Goran Hugo Olsson into a roughly chronological collage that restores a complex human dimension to the racial history of the era.