I'll say upfront I don't know what the finish is.
But after reading handejector's thorough analysis, and studying the photos, I believe it is plated probably with nickel or something akin to nickel. And either, most likely, the nickel bath was tinted with blue or some other close color that didn't work well. Or less likely, was put in a bluing tank after plating but didn't take well at all or didn't stand up to cleaning.
And the gun was shot a lot but the front of the cyl and barrel throat area were never cleaned. It would be interesting to see how well that area could be cleaned up with a good scrubbing. Maybe the same finish as the exterior of the gun would be revealed.
I think the extractor acorn knob is a new part replacement that is factory blued. Or the knob, not being plated, took the bluing very well in my 2nd scenario above.
Just my speculation that's no better than anyone else's.