The present invention provides a method for the blackening treatment of a black Cr-Co alloy plating film, said method being characterized by comprising bringing a black Cr-Co alloy plating film having a Cr content of 1 to 15 wt% into contact with a blackening treatment solution comprising an aqueous solution having a pH value of -1 to 5. According to the present invention, it becomes possible to improve the degree of blackness of a plating film that is formed from a plating bath containing trivalent chromium and has poor blackness, and to further improve the decorativeness of the plating film. It also becomes possible to further improve corrosion resistance of the plating film by carrying out the blackening treatment and then carrying out an electrolytic chromate treatment.