To save time and money, Blum does a lot of his work as a producer — making calls to actors, directors and studio heads — in the back of a gray Ford cargo van that he has equipped with wide plush captain’s seats, two large video displays and window blinds that are nearly always drawn shut. Often, in the middle of a call, the van will stop, the automatic sliding door will open, the aggressively bright Los Angeles daylight will pour in and there Blum will be: at some suburban theater for a test screening of one of his movies ... Horror movies occupy a special place in the hearts of producers. They are cheap, their fans don’t demand well-known actors and the ratio of risk to reward can be astonishing. “Night of the Living Dead” cost $114,000 to produce in 1968 and has since grossed at least $30 million; “The Blair Witch Project” cost $60,000 to produce in 1999 and has since grossed $249 million. Blumhouse’s own “Paranormal Activity,” shot in one house with two unknown actors and almost no crew, cost just $15,000, yet its box-office return since its 2009 release has been $193 million, a return on investment of about 1.3 million percent. ... Because the production cost is low, he can consider other options for movies that don’t seem likely to break big — ones that don’t require an additional multimillion-dollar marketing commitment but could still recoup the initial investment with maybe a little extra as well. Some Blumhouse productions appear on a few hundred screens, often targeted at narrow fan niches.