Printed blacks are not all the same.

Computers display color with RGB light, while presses print with CMYK ink.
On computer monitors, all blacks will generally appear consistent. But on press, different ink combinations can create a wide range of blacks: warm black, cold black, green black, etc. It is important to know the CMYK builds of any blacks used in your project so that you can achieve the results you want.

Some Guidelines:
When black is the text color, we recommend using 100% black (0 C / 0 M / 0 Y / 100 K) for crispest results.
If you have a solid black area larger than two square inches, we recommend using a “rich black” to achieve a thicker, darker and more uniform color. We recommend a rich black color build of  87 C / 80 M / 80 Y / 94 K.
When you have two abutting backgrounds, adjoining color builds should match to avoid color inconsistencies.
If your piece has a black or dark color background, we highly recommend that you add a coating to your piece. Dark backgrounds are more likely to show fingerprints, scuffing and smudges, which the applied coating can help minimize.