As for the SW 99 slide, it's made out of stainless, but it is finished black.
FYI, stainless is actually not the best matierial for guns. It tends to be a little more malleable than plain ol' carbon steel. The anti rust aspect of stainless steel is a moot point with everyday gun care anyways. Also, even stainless steel can rust given the right circumstances. The stainless they use for many firearms, though I don't know the grade, is modified to make it stronger (which I believe actually inhibits its anti rust properties anyways, though not to the point where it doesn't have any anti rust properties.
That's why you won't see me putting 100,000 rounds through my stainless PPK, just enough to practice and ensure it still functions reliably.
-stunks