Block was the youngest of three boys born in Chicago to a Catholic mother, Theresa Lupe Block, and a father of Jewish descent, David Julian Block, a chemist and electrical engineer. (His brother Rich became president of an industrial laundry and his brother Bill was a newspaper reporter for the Chicago Tribune and later for the Chicago Sun.) He began taking classes at the Art Institute of Chicago when he was eleven, and adopted the "Herblock" signature in high school.