In film, Blackmer is remembered for his more than a dozen portrayals of U.S. President Theodore Roosevelt and for his memorable role as "Roman Castevet" in the Academy Award-winning 1968 Roman Polanski film about urban New York witches, Rosemary's Baby, in which he played an overly solicitous neighbor.
In 1919, Blackmer played a major role in the strike that led to the formation of Actors' Equity Association.