I highlight my black by adding blue and eventually bone for the last highlight.
Don't use white or light greys.
If you have acces to P3 paints: coal black is an excellent midcolor for highlighting black and then you can still add a soft light color for the final highlights.
mahon is right. i think ark also glazed a few other colors in, especially on the lower parts of the feet, the weapon (brown). EDIT: and apparently on everything else that faces down ^^
i agree with tmz on the part that a bit of blue sometimes looks good in black, but disagree on the grey part (ark's black e.g. looks good even with white edge highlights) ^^ but that's personal preference really
in general i like highlighted edges on black parts, but as Trov said - they're not a necessity
and yeah, there's a different color on the bottom surfaces. i just simplified it a bit
I think painting black using simple black is not the best idea. "Real Black" usually has "suggestion" of other color. Mostly it is very dark blue, green or brown. So good idea is adding a little drop of dark blue, and highlighting with blue+black mix. Real black save for dark shades and "bottomless pit effects". The same is with white, it's good for light blinks and eyes.