1) Use a Rich Black
Are you referring to the dullness of the black? If so, then you should be using a rich black (also known as built black). Rich black is a created by using a CMYK formula that adds C, M, and Y to 100% K to result in a much deeper black. There are many different formulas (look it up on wikipedia for a good set of rich blacks with different "temperatures").
One build that I use for digital printing is:
Since a lot of digital printers lean toward the magenta side, this blend gives a more neutral tone.
WARNING: There are dangers to using rich black, both on offset and digital printing. Too much ink coverage will often lead to offsetting (ghosting on the backs of sheets) on an offset press. On a digital printer, having too much ink coverage can lead to the ink cracking and/or flaking off of the prints.
2) Make it Glossy
You may also consider using a coated stock and/or adding a varnish coat to add a luster to the prints. A thin laminate may also work well with these, but laminating is usually more expensive.