Steel does not have to be heated red hot to blue it. Commercial bluing, or blackening if you will, was long done to hardware screws and other parts. It was done in steam at 900F.
The idea is to get a fine-grained oxide scale that will not flake off.
Fundamentally, one gets this by heating the steel in an atmosphere with very little oxygen. Without trying to explain thermodynamics in which I am rather dumb, if the atmosphere is 100% steam at 900F there is enough oxygen available from that H2O to oxidize the steel slowly.
If you heat your barrel dull red out in the open, in mostly hot air, it will get a dull gray scale that tends to flake off. Ask your favorite blacksmith.
So Mr. Brooks has a lot of CO in his trench with that hot barrel, and the scale forms slowly, is fine grained, and sticks.
Colt percussion revolvers were blued in some manner with hot charcoal and some really tough guys rubbing the steel with fish oil. They got the metal hot, in some atmosphere with a lot of carbon monoxide and little or no oxygen. I guess the fish oil ate up whatever oxygen there might have been.
The pretty "temper blue" that you get by heating maybe 550-600F is very thin, really it gets the blue because its thickness is similar to the wavelength of blue light. Guys who understand optics would call it an interference film, much like the colors from a little oil on tolp of water. It is not at all durable.
Time for a hamburger & 23oz of my favorite malt.
