Bluing is a subset of black oxide coating. Bluing converts the surface of steel to black oxide (Fe3O4) brought a passivation process. I did a ton of iron oxide chemistry work back at DuPont to switch iron oxide between the various states and back again. Fe2O3 to Fe3O4 to Fe4O5...and back.
We used it as a way to provide oxygen to chemical reactions that could be perpetually regenerated.