IRON/STEEL OXIDIZATION OR FINISHING?

[Flavio]

Hello guys, i would discuss a question with you, if you are interested.
Often when we buy a white weapon, especially on Ebay, we receive at home a sword or a dagger that is rusty and so we clean the blade from rust, but after this we can find that the blade is black. Now, and i speak especially about african weapons because is the field that i know, in many good books the authors write that african blacksmiths embellish the blade giving a burnishing by burning palm oil, and the blackened finish will never loose its color. My question is: the black on the blades is always a burnishing, or the iron/steel oxidization can provokes this result? In other words, could the black color be a fase of iron/steel oxidization (and so the rust a second fase)? And if that is true how we can distinguish the two cases?
Thank you to everyone who wants to participate.