I haven't a clue how it is done but I am suspect the process is quite similar to how you chrome plastic. Which btw, already includes nickel plating. (copper -> nickel -> chromium). Good chrome parts need only have the chromium layer removed exposing the nickel layer and they go from there. Raw plastic need only be made conductive (the copper layer)
At least thats what I have learned in about an hour of googling and browsing websites (mostly searching for black chrome platers and identyifying those that do plastic). Once I found the company, I've learned that true black chrome is pretty much like black powder coat (some Kawasaki m/c's in the 80's had blk chrome exhaust). The 'black chrome' (like on my old bicycle) and what a lot of people calll black chrome is actually black nickel.