Gunsmiths have used bluing for decades to give steel parts that characterstic dark finish. Nowadays, many different types of cold and hot bluing techniques exist - some can be performed at home using a DIY kit, others require special equipment and a professional hand.
It appears that oil hydraulics can also make a contribution to this proud craft! On these pictures you can see parts from a Sauer Danfoss series 90 closed loop pump. The peculiarity here is the gorgeous black finish ALL the internal metal parts got as a result of extreme overheating and some weird reaction with the hydraulic fluid. Unfortunately, I have no information about the type of oil used in this particular hydraulic system, but I can tell you that the black coating is very uniform and very resistant to abrasion.
I believe this bluing technique falls under the category of "requiring special sophisticated equipment"...