Yes I aware that flame hardening is frowned upon by some here on this forum as is cyaniding. If used correctly, however its results are superior to cyanide hardening.
If one were restoring a firearm, then there is but one choice and that would be to anneal the parts to remove hardness and temper and use the methods employed at the time they were manufactured, ie bone and charcoal pack hardening. Bear in mind the coloration of the steel is secondary to the purpose of hardening it. A welcome side effect if you will.
If one is attempting not to restore but to place back in service a tired old worn piece, flame hardening would be preferable to cyaniding. It is safer and yield better results.