I once bought a Walther PPS with that weird blue frame. I checked around and found that I could dye the frame using Rit black dye. As I recall, it was simply a matter of stripping the frame of all components and suspending it in a pot of hot dye and salt. After it's completed, you wash it very well and re-assemble. I think you can find the full instructions on the net. I believe it was under an H&K heading somewhere.