The problem with black is, basically, that you shouldn't be painting black at all. There's three reasons for that :
a) Look around you and try to find an object that, in good lighting conditions, is really really black. I bet you'll have trouble finding one.
b) A black real life sized car might look good. The exact same car in miniature scale will look less good. Our perceptions works that way, not just with black but with every colour. This is why we use such extreme, unrealistic highlights. And if you have a look at larger miniatures, like busts, you'll notice how, as the scales get bigger, the extremity of the highlights and shadows get smaller. Why? Because on larger surfaces, light itself takes care of the highlights for you. In other words, just because the real life car is pure black, it doesn't mean you should be painting your miniature version of it in pure black.
c) Pure black has a unique problem. You can't apply shadows to it. This is the equivalent of losing one dimension of a three dimensional object. White has the opposite problem, where you can't apply highlights to it. They're both doomed to look crappy. This is why GW highlights the white Tau scheme with chipping/rust.
For all these reasons, what you might want to try out (and this isn't a magic recipe), is going for a neutral dark grey. This can mostly be achieved by mixing black into a neutral grey paint. The "neutral" part is kinda key.I must warn you though. Figuring how much grey is too much grey is tricky.
Also, another thing that makes a lot of difference with black is reflectiveness/glossiness. Try a glossy black and then try a muted black (just apply lahmian medium on top of a black paintjob) and see which one looks better FROM A DISTANCE.