>>548273
More of a fuckton of polishing then letting the gun sit on a frame in a rotating drum with a ton of charcoal that is heated up to around 600f. That is how they blued all their guns before WW2. Carbona or Charcoal bluing is what its called, honestly there is a reason why people use salts to blue. Modern guns are salt blued after a ton of polishing. A History of the Colt Revolver From 1836 to 1940 is the book I was referring to.
>>548275
Generally its still hand done in regards to firearms. Pythons like in OP's image are polished to a near mirror finished before being blued.