I use a dark grey and then give it a wash in nuln oil or a similar wash. Black is rarely black looking because of light hitting it so I never paint with black paint.

I’m the same. Paint Vallejo German Grey and wash with Nuln oil. I’ve tried lots of methods but that’s my go to. The other one is highlight with a mix of dark prussian blue and German grey, then Nuln oil.

Similar to the posts above, for 28mm I pick a dark colour and stick on a heavy black wash. Base colour varies though. A dark grey (I typically use Vallejo London Grey) is good, but I also like Vallejo Dark Sea Blue and Vallejo German Camouflage Black Brown.

For smaller scales I tend to use a matt black base coat and just highlight with dark sea blue.

Personally I start with a black base, then highlight with greys starting at almost black up until very light. (Amount of grey layers depends on how much time and detail I’m willing to put in).

Then add black glaze (I mix vallejo black with lahmian medium).

Make sure glaze isn’t too strong as its best to add several light glazes.

This will help fade the greys to black but keep the highlights.

My go-to black recipe is to paint with Corvus Black (its actually a very dark grey), highlight with Mechanicus Standard Grey, then wash everything with Nuln Oil to make the highlights darker again.

I’m not sure what paint range you use, but the usual colour in Citadel is Rhinox Hide, so something that matches that colour should work. You’ll want to thin with medium, but I’d recommend getting the consistency closer to, say Contrast, than to a typical wash/shade paint.