When you're printing in full color, the black is a "process" black and can't be so dominant that it swallows all the other process colors up. When you design a printed piece with a black box or a solid black background, it is recommended that you use what's called a "Rich Black".
You can see from the image above that there are two black boxes. One is using black only - C 0 M 0 Y 0 K 100. The other is using "Rich Black" C 60 M 60 Y 40 K 100. You won't see much difference on your computer screen due to the nature of how it displays, but you can see the difference in your design software. Take a look at the color boxes in the top middle. The dark black is the "Rich Black" while the bottom is black only.
DO NOT use "Rich Black" for lines or text as it may cause registration/printing problems, but it is perfect for large areas, backgrounds, and areas that need to be solid black.