The black was a very, very dark gray. I bet you wouldn't be able to tell the difference from 3 feet away, or under low light.
There is one upside to the dye process - if the color doesn't work out, you can still paint it black without having to strip anything. Good luck.
I dyed a white pickguard red, and it turned out fine. YMMV.
As for the process, re-read the post below...
If you check out any of the available YouTube vids for using dye on RC parts, you will see that items are often left in the dye bath at a low boil for a few minutes. I do not recommend doing this with the pickguard, as too much heat could easily warp it. IME, you need the dye bath to be just below boiling to ensure that the pickguard remains as flat as possible.
Again, YMMV. If you've never done something like this before, be prepared to chalk up any potential mishap as a learning experience.