How to thin blueing?

[Sunray]

Citric acid is what's in vinegar. It'll strip any bluing to bare metal quickly. 0000 steel wool without any oil, maybe. Rubbing with a hunk of leather might work better though. Leather being what makes handgun lose their finish along the edges.
"...done in a blueing(SIC) tank..." That's a different process/chemical from cold bluing. Cold bluing is more colouring than protection.
Those parts are very likely new replacement parts. There's really no way to 'age' it to match 120 years of life.