I'm no metalurgist, scientist, nor expert. But I'd take a guess by skimming through the nighmare of physics classes past info that it's some form of what we know as Carbonia Bluing.
Not the same process,,but the same idea.
Oxidation of the metal in the absense of free oxygen(atmosphere)
It talks of temps in the 1000C range.
Mr Gores internet tells me that's 1830F. Much higher than anything needed for Carbonia or charcoal bluing. It's even much higher than we ever ran case hardening. Exactly 400 degrees higher.
All just a guess on my part. Gave me a bit of a headache to look thru it,,but I gots to know.
Time for a drink and then another coat of LMF it is on a set of shotgun parts.