A lot of people (including myself) have had trouble painting black. This tutorial describes my take on painting black.
Black is black
Well, for starters the simplest way to paint black is to just leave the area black - or most of it at least. Adding too many subtly blended highlights just makes the area appear grey.
The best take on highlighting black is to add fine and sharp highlights only to the very edges. However, compared to highlighting other colours highlights on black often needs to be somewhat lighter - something like Chaos Black + Shadow Grey, then Shadow Grey on its own, and final highlights using Shadow Grey + Spacewolf Grey (maybe even pure Spacewolf Grey in a few places).
Having painted the highlights I often find that the area has turned somewhat grey despite the fact that I have been very careful. Therefore I glaze down the areas using a thinned down mix of Chaos Black and inks. Using this technique you can actually make the area "darker" than plain Chaos Black as the ink adds to the depth of the colour. Whatever ink you choose depends on the overall colour scheme on the model.
Describing the mix of Chaos Black, ink, and water is always hard to do, but it should look like a very dark (almost black) version of the used ink colour and be somewhat glossy (especially if you compare to some dried up Chaos Black on your pallette). Also, it is always better to paint two layers of thin paint than one layer of thick paint. This way the colour can be built up using multiple thin layers blending the shading into the creases of the model. Try to keep the glaze where it is supposed to go and avoid painting over the final highlights. That said, you can always go back and redo the highlights if necessary.
Various shades of black - a selection of examples
Like all other colours black isn't really just black. Black can be highlighted, shaded, glazed, and varnished to represent many different kinds of material. The following examples show some of these possibilities:
My Barbaretta conversion features both black armour and a black rubber suit. The armour is painted using very limited bluish highlight and gloss varnish to add an armourplate-like finish. To contrast the black armour I have purposely made the rubber suit somewhat grey and used only "real" grey by adding Skull White to Chaos Black. Also, again for contrast I matted down this area using a matt varnish.
The cloak on this Witchhunter has been highlighted by adding Bleached Bone to Chaos Black rather than using a shade of grey. Also, the cloak has been glazed adding purple and red ink to Chaos Black.
My Warmachine Manowar has a classic old School 'Eavy Metal-style black armour. Using various blues and bluish greys for the highlights, glazes of red and purple, and keeping the highlights very sharp and limited to the edges only results in a very strong and deep black.
"Army painting"-style black
A good tip when painting large armies is to simply leave the areas such as boot, gloves, horses plain black and instead concentrate on the models main parts: e.g. weapon, shield, the head, the horse's barding, etcetera.
If you do want highlights on every trooper I find that two or three sharp highlights on areas such as the tip of the boot, the knuckles of the glove, etcetera can do the trick.