Black Oxide.
Whatever it was, it came out sharp!
I love the before and after of that gun. I especially like looking back at that old thread and seeing how discouraged you & your pop were when you first got it. He chucked them in the basement and you were ready to part them out.
Heck Nightstryke, that doesn't look all that bad to me. Even the stock looks pretty solid!
I haven't had to deal with much rust, but the bluing wear I can certainly relate to:
Thankfully, I live in a dry climate and haven't really needed to do anything to it beyond a good cleaning and oiling.