This Canadian company was established in 1881 and at some point became owned by Dominion Coal Co. which had been established in 1893. Its flag, as shown by Griffin 1895, Reed 1901 and 1912 and Lloyds 1912, was red with a black diamond. The livery was also used by Dominion Coal Co. who were ship owners in their own name and by another of their subsidiaries, Dominion Shipping Co. Ltd.. Dominion Shipping Co. Ltd. was always a Canadian company although in 1934 the British company Donaldson Bros. & Black Ltd. took over control [presumably this meant as managers and seems to have ceased after WW2] and Lloyds through to the end continued to show a Canadian address. I have checked my sources of Brown 1951 and Brown 1958 and Stewart 1953 and Stewart 1963 and they all say Canadian. Their last ships were sold in 1965.
Brown's Flags and Funnels of British and Foreign Steamship Companies, compiled by F.J.N. Wedge, Glasgow, 1926 report a yellow flag, in center black diamond; in all corners black "BDSC".
I assume that this was an error and Brown 1926 ascribed the flag which actually belonged to the American company Black Diamond Steamship Corporation of New York to Black Diamond Steamship Co. Ltd. of Montreal.