Jon;
I don't really know that much about what all can be done with the Cyanide process. To harden though requires the part be above its critical temperature prior to quenching. Even presuming that Cyanide can be used to produce color below this critical temp, it would still I suspect require a temp much higher than I would want to submit a gun of mine to, whether a low carbon or higher carbon alloy steel. Either way you are going to end up with that soft surface.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra