Originally Posted by Hapworth

"Bluing" or "blued" refers to a variety of specific chemical processes for controlled rusting of carbon steel to effect a blue appearance and minor abrasion and corrosion protection.

Stainless steel does not take to the process and the term "bluing" doesn't correctly apply, irrespective of whether stainless has been darkened through one of the several other processes that can achieve that.

As mentioned, nitriding (aka: ferritic nitrocarburizing; quench-polish-quench; trade names like Melonite, Tenifer, Hard Hat, etc.); Cerakoting (and similar spray-and-bake); Ion Bond (PVD); and an oxidizing process that turns the chrome in the stainless to chrome-sulfide, blackening it.

None of that, however, is bluing.

Of the processes that work on stainless, I believe nitrocarburizing, Ion Bond and perhaps chrome-sulfide blackening are amenable to metal preparation producing reflectivity from matte to high lustre, depending upon the polishing applied to the stainless; spray-and-bake does not.