If you have a nearly black background and want it completely black, you can easily adjust this in just about any RAW converter. Simply move the black point to get the desired effect.
If you shoot JPG then you can do the same with levels in any half decent photo editor.
If you need more control you can use a custom curve instead - either in your RAW converter or in any decent photo editor. I would recommend this method.
Burning like James suggest provides more control, although I much prefer to burn using curves on a separate adjustment layer because it is even better and much more flexible. Unlike the "burn" tool it offers total control over the effect. Change your mind? Too dark or too bright? Simply adjust the curves!
Just search for dodge burn with curves and you will find lots of tutorials.
