Had two sons - Jonathan Blackmer and Brewster Blackmer.
His widow became entangled in a battle with Donald Trump during the 1980s. Trump wanted to sell the apartment she rented under New York City's Senior Citizen Rent Increase Exemption Program. Trump wanted more than the $1000 a month restriction and wanted her to sell the apartment. Eventually, in 1998, Trump was allowed to turn the apartment into condos, subject, however, to her tenancy. (Suzanne Kaaren) Mrs Blackmer did not receive any money as a result of the legal battle with Trump. Trump was enjoined from all attempts to evict her from the 100 Central Park South Apartment where she remained until her death in August of 2004. The Blackmers rented the apartment in 1948 and maintained a residence throughout their lives.
While he portrayed Republican President Theodore Roosevelt more times than any actor on screen (eight in all), he himself noted that he was, ironically, a Democrat.
Won Broadway's 1950 Tony Award as Best Actor (Dramatic) for "Come Back, Little Sheba."