Gunsmiths have used bluing for decades to give steel barrels that
cool looking dark finish. Nowadays, many different types of cold and
hot bluing techniques exist on the market. Some of them can be
performed at home using one of those "do-it-yourself" kits, and
some require special equipment and a professional gunsmith to be
It appears that oil hydraulics can also make a
contribution to this proud craft! On these pictures you can see parts
of 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" ones...