I would paint a coat of watered down black over the black primer.
That said - a method I use for black for gaming pieces is to simply paint the black areas black. Spend the time highlighting and shading the other areas and the black should look fine unshaded/highlighted.
Once the figure is varnished it isn't something that stands out as unfinished. Now if you are going for a display level figure you may want to work some greys, browns, or blues into the black to highlight.
I do a couple of hilights (on everything) in the normal way. The trick with black is not to overdo the hilighting and to use as neutral a grey as possible (I mix black and fortress grey) when hilighting.
That said I only really paint large areas black, small surfaces not really benefiting from the hilighting the same way.
Do it right and you too could have miniatures that look like this:
(although almost certainly worse than Dallimore's)
The trick with painting both white and black is that it is not a simple problem of how to highlight these 'colours' properly and have them come out perfect - it's all about shading instead.
A good way to paint black is to start with the base undercoat, black, and then to mix a tiny bit of bleached bone into the colour on your pallette (use a pallette, if you dont already!!!), and a little water to thin it out. Then you should 'shade' the parts you want to be brightest, such as the topmost parts of the armour plates on a marine.
Then mix in another small amount of bleached bone, and 'shade' again, this time filling in a little less of the top parts of the mini (essentially, you are filling in a smaller part that which you have already shaded).
Repeat until you get a bright enough maximum highlight to suit your style, and there you have it! Try not to go too bright, it can look good but is hard to make work.
I choose bleached bone because it tends to make the softest addition to black for shading, everything else seems too harsh.
It is similar with white, where a good start is to basecoat the to be white areas with fortress or codex grey, and then shade up by adding small amounts of skull white to the mix, until only the smallest part of the area you are painting is actually skull white. Trust me on this: it will look more white than actual white this way, and will come out much smoother than just painting white over, say, astronomican grey.
Anyway, here's some white and black I painted using this method (although there isn't much of either here, hopefully you can see what I mean, even through the shine of the flash from the camera):
Bleached bone is nice for hilighting black but I prefer kommando khaki though, it gives a similar finish really; I just prefer the tone. (I love kommando khaki you see)
Space wolves grey and shadow grey are good for white too as they have a slightly blue tone to them, which looks better under most lights.
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