Painting black isn't at all easy. The original highlights I had, and the inking didn't work out at all. So I had to start again, add a new basecoat of black and some grey.
Since I wasn't sure how to fix the initial issue, I decided to head the route I already used for the ZEUS, using Boltgun Metal and give it a good drybrush.
This time I used Adeptus Battlegrey for the outlines.
It is a lot lighter than the previous grey-mix, but on top of the drybrush it seems to work out nicely.
It is just hard to capture in the photos.
But I think you can get the idea.