Painting black uniforms can be broken into 5 levels.
A base color, midtone highlight, and a highlight, midtone shadow, and shadow.
For a base color of black, I use a very dark blue or a black mixed with blue to create a dark blue. I then paint my midtone highlights with the base + my highlight and then for my highlight I use a very light blue or light grey.
To paint my shadows I then mix my base + shadow, and for the darkest level of shadow I use black or a black lightened ever so slightly to make it just above black.
This gives you a full range of color and provides depth to the figure. Any road dust, grime, etc can be drybrushed if necessary or if you feel that you don't have enough contrast in your highlights you can drybrush your highlight color again.
Paint highlights before shadows as its easier to paint dark over light than light over dark.
By using a no black base coat, you get that depth but it allows you to also use black as a outlining color to define lapels, pockets, buttons, medals, seams, hems etc.
If you find you have too much contrast you can can knock it back by making a glaze of oyur midtone shadow or base coat and do a wash over everything. This will dull out the contrast and shorten the levels of tonal changes on your figure.
With acrylic the more "levels" you use between tones (think of laying acrylic down in layers like looking at a topographical map) the smoother the transition between those levels.
If you are using oils or enamels, you can do the same thing with the basecoat (even with oils I use a base coat of acrylic. It allows me to start doing some nice shading and highlighting without worrying about breaking up the base coat). Then a highlight can be blended with the shadow in a physical blend to make the tonal transition.