Black Oxide, Chemical Black, Oxidizing... these terms all refer to the process of forming a black iron oxide on the surface of ferrous metals.
The black oxide process is a chemical conversion coating. This means that the black oxide is not deposited on the surface of the substrate like nickel or zinc electroplating. Instead, the black oxide coating is produced by a chemical reaction between the iron on the surface of the ferrous metal and the oxidizing salts present in the black oxide solution. The result of this chemical reaction is the formation of black iron oxide, magnetite (Fe3O4), on the surface of the metal being coated. Oil is then applied to the heated part, which seals it by “sinking” into the applied porous layer. It is the oil that prevents the corrosion of the workpiece.
There is no significant dimensional impact, the blacking process creates a layer about a micrometre thick.