As my friend PCRCCW said...
The "special two-tone" slide is actually 4140 carbon steel. The frame is stainless. The lubricity of the two types of steel sliding over each other is just outstanding. I usually *do buy all stainless steel handguns. A blue handgun is prettier to my eyes, however summers in the Ohio Valley are HOT & STEAMY. For example, sometimes, I prefer to carry non-stainless blade pocket knives. However, a non-stainless blade rusts in my pocket in about a week. I have a little better luck with "D2" non-stainless blade steel, but it just puts off the inevitable rust for a couple of weeks---then, there it is.
I'm hoping the black coating and a little extra maintenance in the summer will keep the rust away. After all, I've carried blued revolvers through the summers here without rust. I'm hoping for the same...no, I'm hoping for actually BETTER success with the two-tone MK9. I haven't seen many Kahrs with black slides. (Other than the "Black Diamond" stainless slides.) I have seen some with a black frame and a silver stainless slide though.
KR