The very best method for bluing and blackening is to take a 50/50 mixture of sodiun nitrate and potassium nitrate and melt it in a container to between 600 and 650 degrees; you simply immerse the item in the bath until the desired color is reached, which is a matter of minutes. The part need not be clean, as the nitrate salts will burn off any oil or fingerprints that may be present, and this makes a very rust resistant finish. This was titled US Armory method of bluing in an American Machinist technical article. I first saw that it worked when I was doing heat treating in the shop where I apprenticed back in the 1960s, where they used a tempering bath that had a set point controller to regulate bath heat; I noted that articles that were ground after tempering and may have been tested as being over the desired hardness and subsequently re tempered at the higher temperature came out of the bath with the colored finish; this happens while the piece is under the surface of the bath, and has nothing to do with temper colors that would be created by exposure with the air when heated. More recently I found the "recipe" in an American Machinist Shop Notes article. When my apprenticeship shop closed (Kaiser Steel Napa Plant) I boughtall the heat treating equiptment including about 1/2 barrel if the salt mixture. This mixture is the lowest melting salt mixture known, it will stay liquid down to about 275 deg. F.