The OP was black oxide vs zinc plated. There are also other ways to get black including zinc chromate. That can be steel or alloy steel, stainless would be neither.
(OK stainless is a steel alloy but you don't typically plate it) Personally I kinda like the black. I wouldn't use stainless unless I happen to have them on hand. In terms of typical FRC it is more of a preference than an engineered decision though.