Black oxide coated hardware is often made from high strength alloy steel which has been hardened some. Not *****'s heart hard, but harder than typical butter soft screws.
The black ox is used as a coating because it is low heat and doesn't affect the tempering of the metal. It provides little corrosion protection, but does hold a bit of oil to keep them from rusting in storage..
Stainless screws are usually 18-8 (very similar to 302 or 304) which is soft, but work hardens fast. It can be tough to drill by hand because the first time the chip breaks the surface hardens and won't drill anymore.
