CMYK (Cyan, Magenta, Yellow and Black) are the colors used in the printing process. Within these colors, there are certain colors when printed look dull or different. These colors are usually black and blue.
How to produce pure rich black
In order to achieve a rich solid black you must enter certain numerical values to make up for the limitations that CMYK entail. See diagram below:
You couldn't get a rich black by just picking any black from your color pallet. You would need to adjust your black in order to get the richest darkest black. The picture on the left side has the CMYK values of C 60 M 40 Y 40 K 100 which will give you the purest black.
How to prevent your blues from printing purple
The same concept applies to blue, although there is no dullness or lack of richness. With blue, its the hue or the color that shifts. When printed blue has a tendency to appear as purple. To remedy this make sure to leave at least 30 point difference in your Cyan and Magenta values.