The
GW "basic highlight" for black is dawnstone then administratum grey. But the answer is that it depends a lot on what you're trying to highlight.
For instance, these guys are highlighted with Dark Reaper first, then Fenrisian Grey. This gives it a hard, bluish highlight:
On the other hand, the back of this cloak is wet blended entirely with greys to give a soft, black cloth look:
Note that you can't really achieve a nice shaded black cloak with washes. It just doesn't work well. You could glaze it forever, but a wet blend is just much easier (and faster).
On this scout, I wanted to NOT introduce any blues, because that would diminish the "urban grey camo" scheme, so I highlighted with the standard dawnstone/administratum: