For anyone interested in a technical, historical perspective, my research has revealed that "black oxide" - the solution and the term - was invented by E.F. Houghton & Company in late 1941 in direct response to War Production Board restrictions on nickel- and chrome-plating. Like triple-alloy steels*, invented by a government-industry-academia consortium in 1941 and 1942, it did not exist prior to WWII. It is a high alkaloid solution finish baked on at 295*F. In my experience, no mfgrs made more extensive use of it than Walden-Worcester and Sherman-Klove. You will find other wartime black oxide tools, but not nearly as often or as common as Walden or S-K.
* I have a long-held theory that Vitalloy and Zenel were secret triple alloys.