Deathwatch, painting black models

I have a full Ravenwing army and also a small deathwatch force. It is a nice color for an army, but getting the shadows and highlights with black is tricky. Often, the finished model looks like more like painted dark grey, instead of pure black.

A few models I painted before achieving the technique I am satisfied with :

After painting a few dozen models with this color, here are my conclusions :

The trick seems to be in leaving larger pure black areas compared to painting and shadowing another colour (like red, or blue).

The detailed step by step for my painting technique for my latest "black" models :

- black base coat from vallejo black polyurethane (airbrush). I found that the black spray from citadel looks instead like a very dark grey. Before I got my airbrush, I often repainted my base colour with vallejo black when I was using the black citadel spray. Vallejo black is a nice, mat black that I find more deep than the citadel black (but it may be a personal bias).

- very light zenital highlight with vallejo dark grey  model air. It should not be too strong, otherwise it will not look like a black model.

- Edge highlighting of the zenital edges with the following colors (vallejo air model) in this order, from dark to light : dark grey, dark blue grey, blue grey, light blue grey, light grey (almost white). On the smallest areas it should be possible to skip entire steps. The trick is to do this on an area maybe of half the size it would be with another color.

If an edge seems too strongly highlighted it is possible to mute it down with some black wash.

Finished model :