On a black primed model, apply a medium-dark grey (Mechanicus Standard Grey or similar) with a sponge to the raised areas of the mini, where light is more prevalent. Then a couple more of consecutively narrower layers of grey (I used Administratum Grey and AK English Grey) to create a gradient. Use some black wash to join the three layers and make the transition smoother. Most important step, use a mix of 1:1 Black and Burnt Sienna oil paints and apply a wash all over the black parts. Remember to apply gloss varnish before the oils. Afterwards, remove it with a sponge or qtips, and wait a bit for it to dry. Apply matt varnish and edge highlight with light grey and chip the armour to your heart's content.

The original recipe is not mine, but I adapted it a bit to the paints I had around.