As noted above, it's called "case color", and it's meant to look that way. Traditionally, it was the result of a carburizing surface hardening process called case hardening, which forms a thin layer of of hard wear resistant steel (the "case") while leaving the interior steel softer.
It's questionable whether S&W truly case hardens these modern MIM parts, or if it is just a surface coating made to give the appearance of case hardening. I'm inclined t believe the later. Either way, the parts seem to work fine.