Some of the cold blue solutions contain copper selenide. I'm guessing the copper reacts with the gold causing the coloring. (It has been way too long since high school chemistry to be sure of that)
The Glock slide I engraved was 416 Stainless that cut as nicely as any Stainless that I have engraved. I do not know the engraveability of 17-4, but suspect it could be hard since 17-4 is used for items that require hardness and corrosion resistance (Wikipedia), however it is...