Those black spots on the metal. What is that?

The process was known as cold rusting. Rust (red) would actually be formed and would be polished off leaving the black oxide. To get a uniform surface, the rusting and polishing have to be repeated a number of times. The metal needs to be polished when the rust layer is light. If left to rust to long, pitting would occur. When I was a kid, I used to do this with knife blades. It took some time to get a uniform finish but, once you did, it was quite resistant to further rusting. I thought that I had come up with quite the process. Later I learned how old the process actually was. Reinventing the wheel, as it were.

Interesting. Didn't know that. Thanks for the explanation.

Generally known as "browning" in the UK. Still fairly common amongst gunsmiths, especially as legal UK shooters are now generally firing antique black powder kit.

Very common here at one time also. My grandfather's old Damascus barreled shotguns that I grew up duck hunting with were not "blued", but "browned".