A patina is actually a type of oxidation/rust. If I remember correctly, patination, or "black rust" is Fe3O4, and normal oxidation, or "red rust" is Fe2O3. While "red rust" has a damaging effect on metal because it has a tendency to "eat" it, patination is an oxidized layer of metal that stays on the surface, which helps to prevent "red rust" from taking hold of the steel.
I'm no chemist, but I think that's pretty much correct. If I'm wrong, someone feel free to tell me so
As far as what will make a patina develop--pretty much anything. As mentioned, acids are what cause patination, while moisture causes rust. There are acids in the oils on your skin, in the leather of your sheath, and in everything you cut. As long as you keep the steel dry and oiled, the patina will develop slowly, and "red rust" should not develop at all. If you want to force the patina, I certainly won't hold it against you--I've done it to several blades myself, although I prefer the look of a natural patina. Sticking the blade in warm vinegar, or into an apple, onion, or potato, or covering it with mustard, are the most popular methods.
~Noah