Thanks, Butlersrangers!.  I was aware that the receivers and some other parts were case hardened but quenched in oil rather than water.  The color variations in the common case hardened steel is due to intentional uneven quenching.  The different colors represent steels of different crystalline structures.  I was always under the impression that the color from oil quenched case hardened steel was a uniform outer layer of steel of a particular crystalline structure that had a gray-black color.  But I was told recently that the dark, uniform color was due to the burnt or coked oil.  The coating theory seems feasible as a uniform layer of steel from the quenching would be thicker and more durable.