You might try a REALLY harsh solvent, but even then...
The "blacK" is unfortunately the nasty stuff from the combustion process that has literally been baked onto/into the metal.
You could try soaking a scouring pad in xylene or toulene, those are as strong a solvent as you can get. They are sometimes relabeled and/or blended to by slightly less carcinogenic, but these are really unheathly. You have to use chemical proof gloves and be very careful to avoid breathing the stuff.
You might find them in "laquer thinner" too.
Solvents MIGHT work, but I would not count on it.
The only sure way to remove the "black" is with a bead blaster, and really that is not practical to do very often, as even a skilled operator will eventually remove some metal and thin out the tip.
Of course with either the harsh solvent or the blaster you need to remove the tip -- the risk of getting solvent on a painted surface is too great, and blasters for even the smallest parts still have a lot of overspay.
It be more cost effective to just replace the tips every few years.
