You could also use permanent marker on the, wash off the excess with an alcohol pad and gently apply a little see through epoxy. Not for the purists, but it will work.
Testers model paint works great. Use the Flat Black. I have tried this technique on a few user grade Slims and a Fatboy I had on hand and it worked great. I have let them set up and have been testing a couple with simulated use and it holds up and looks great.
Black nail polish, works well too. I did not even have to buy it, just "borrowed" it from my teenage daughters. Now why are they painting their nails black is another unanswered question!