I've seen black minis (Space Marines actually) done well by doing very specific edge-highlights using greys.
But the Relthoza models are SO detailed that I think you have to start with the drybrush method - base in black, drybrush on extensively with a pure white (or blue-grey, depending on goal effect).
For wash, something like Nuln Oil can often seem brownish, so choose carefully and test results on small areas or models first.
I usually prefer to pick a contrast shade, like green or red, and ink-wash in that over the white. This can look flat though, so be careful.
Then go in and pick out details like windows and gunports.
If you want it to look really black though, ala Stealth camo, you'll want a darker drybrush color like a grey, and then a true black wash. Maybe a gloss. Your only friend for causing depth might be natural light reflections to indicate depth, such as a gloss offers.
If you're really patient, picking out recessed details and highlights in darker shades can give that depth help as well.
