While we are discussing, any process that causes a fine grained, thin film of black oxide to form on the surface of polished steel will appear blue. As the grains grow larger and/or the film thicker, the apparant color will shift from irridesent blue to black. "Heat colors" are formed when polished steel is heated from around 300 F to 700 F (2p, fill in the colors vs temeperatures as my text book is at home, please); a fact important to tempering hardened steel. "Fire blue" is sometimes used on gun parts by heat coloring polished pieces. Another process than can add confusion to coloring is the molten salt process. It is based on salt (NaCl) melting at a temperature that promotes "fire blue." However, it can be held long enough to give a much thicker film than the usual heat colors; Winchester used it extensively. The film is thick enough that it will, in later life, sometimes actually flake (the reason some old Winchesters have bare spots of steel).
