Nitron finishing will blacken stainless steel through the use of oxidizing type blacking compounds.
This process produces black coatings on stainless steel and alloy steels by a "conversion" process. The chrome in the stainless steel is involved in a chemical reaction with the oxidizers in the solution producing chrome-sulfide.
The application of the black finish is dependent upon the surface treatment of the steel. Bead blasted surfaces will have a satin black appearance while a surface blasted with aluminum oxide will be matte in nature.
Importantly, dimensional changes involved in blackening are extremely small, less than 0.0001 Mil. This thickness of the black oxide type coating ranges from 0.00006-0.0001 Mil depending on the type of alloy being treated.
Blackening of stainless steel not only makes the metal more attractive but adds to its corrosion resistance (96 hours salt spray per Mil-C-13924B) and prevents galling, common in stainless steel metal parts or firearms.