Lewis had four older brothers, Gordon Patteson Blackmore (1872-1941), George Edward Blackmore (1874-1936), James Gairdner Blackmore (1876-?), Edwin Ord Blackmore (1879-?), two older sisters, Jane Gordon Drummond Blackmore (later, Mrs. Granville Sharp) (1881-1942), Eleanor Mary Blackmore (1884-1891), and one younger brother, John Coleridge Blackmore (1888-?).
He attended St Peter's College, Adelaide, and moved to Melbourne Grammar School in 1903.