Well if you have a few decades to wait that might work. Might try some cold blue on a degreased place out of the way, or a spare part with that finish. Might work? If it is zinc instead of manganese finish, it will probably always be lighter looking. Also have read about chromic acid being a part of the process for some military finishes, which is pretty hazardous stuff to experiment with. That might affect the color of the finish though, I suspect.