How to Highlight Black?

I usually do this:
-basecoat with a dark grey ( say Charadon Granite, but it can be any grey, it just has to be veeery dark)
-heavy drybrush with less dark grey (say adeptus battlegrey, but any equivalent is ok)
-light (and i repeat, light!) drybrush with codex grey
-thin highlighting/lining along the edges and raised areas with a mix 50:50 codex grey/ceramite white
hope it helps, cheers
-nasty and reckless washing in nuln oil.

for space marines i highligt the obvious angular parts and also, on the flat surfaces, some thin lines near the edges of the flat area. If you have some raised and well visible borders with edges, just highlight that. Flat areas can avoid the highlightin whit some "cheating", using decorations, decals and gluied accessories to fill space.