Mix black with another colour, only a little to give it a petrol or tinted look. Wash with a contrasting colour (purple if you mix in a little green with the black - work with the colour wheel of 'light' instead of 'pigment' which is what most colour wheels are made from, the shadows are made from light - not colour, after all.)
Washing pure black is like having black hair and dying it a temporary lighter colour without bleaching first - it won't work, the lighter colour (the wash) will sit on the black and you WON'T SEE IT.
Nothing black has black highlights anyway, light still bounces back and black isn't in the colour spectrum.