House flag of CBMC
According to Scott's Caltex book of Flags and Funnels (1959), the house flag of Compagnie Maritime Congolaise is blue, swallow-tailed, with a red cross throughout with a small black, white-bordered disk on the cross center, a yellow lion rampant therein; in the upper left corner, a yellow five-pointed star.
Larousse Commercial (1930) shows the same flag for Compagnie Belge Maritime du Congo, but with a yellow-bordered disk.
However, Lloyd's book of house flags and funnels of the principal steamship lines of the world and the house flags of various lines of sailing vessels, published at Lloyd's Royal Exchange. London. E.C. (1912), also available online thanks to the Mystic Seaport Foundation, shows the house flag of Compagnie Belge Maritime du Congo with a white-bordered disk.
Brockhaus' Konversations-Lexikon (14th edition, 1898) gives a different house flag for Compagnie Belge Maritime du Congo, white swallowtail with a red St. George's Cross, and a yellow crown in the centre.