The burnt oil will work on steels as many other applications will do also.
It all depends upon the quality required and finish.
Hot blacking is the best but operate in region 150 to 160 degrees. Cold blacking as Birchwood Casey and other gunblue kits waste of time except for screws.
There are cold/warm blacking formulas that rust the metal after painting on and have to be wire wooled off between around 7 coats, as used on English shotties.
Theres phosphating also quite rough to the touch.
Other metals will require a separate different process namely aluminiums and titanium requiring anodising, stainless forget it too costly.