I just heard back from the owner of it and they said that "the dark surface is just aged brass patina".
I'm not convinced that it's a bluing pan, though I think that someone made it for heating something. It's probably a one off, and if that's the case then it's almost impossible to guess the exact purpose that the person had in mind.
Rob
You don't use blueing solutions in a blueing pan. You fill the cavity with brass filings, and heat blue the part over an alcohol lamp. The brass filings make even contact with the steel part (like a watch hand) being blued. Am I the only one here that has done it?
Steve R.