Blackening steel can be achieved a number of ways. Blacking or bluing is done primarily for two reasons. The first is the appearance, blackening produces a unique appearance that is not producible in any other method. Second is blackening inhibits rust. The color is caused by an oxide layer which acts as a barrier protecting the underlying steel.
Achieved using chemicals - often selenic acid based- cold bluing and russeting can be achieved without the application of heat and are generally inexpensive. Cold blues generally create a very thin layer that easily scratches off. If the acid or other catalysing compound is not completely removed the armour is likely to quickly rust.
This is generally achieved with oil and a fire or torch. Many people have reported good results using linseed oil. Oil blackening generally produces a thicker more resistant layer to the piece than cold blues and will usually work as a rust inhibiter.