Bluing is just a special paint, so since it was protected it doesn't surprise me it's still there. The marks are interesting, it really makes me wonder about how they made it. I am not sure I have ever used bluing on something I put in a mill, but I can see that some might.

I think there may be some layout fluid left over on some of my fixturing from the '70s. I had some parts machined, but not much of the work I did was even done in a drill press.