It is a surface treatment performed in a warm bath with blackening salt - chemical passivation of the surface:

• a very thin (raughly of 1 μm) black layer of ferrous-ferric oxide is formed
• the oxide layer is relatively durable and well wettable for oil - the process ends with the preservation of parts
• good protection against corrosion and abrasion resistance
• suitable materials for this surface treatment are iron castings, non-alloy and low-alloy steels