[jimsloth]

If your white paint is very good at covering, I'd just slowly work backwards white over black. I use Citadel Skull White for just this reason. Make sure your brush is super clean, shake the paint before opening, use only a tiny amount on the TIP of your brush, if you get more than a drop on there, take a stroke over a paper towel first, Breath in before your brush touches plastic and hold your breath during each brush stroke.

If you need to use black again to work the other way, wait until it is good and dry first. You don't want those areas blending so it is important to only use tiny brushes of paint and be patient.