There are two types of black ink used in printing: standard black and rich black.
The standard black doesn't produce a really deep black color. If used in larger surfaces, they look a bit grayish. Professional designers mix another printing inks with the black ink to get what is called rich or deep black. We recommend a mixture of C=60 M=40 Y=40 K=100 (240% ink Coverage).
We avoid using Photoshop's black value (C=75 M=68 Y=67 K=90) since it goes above the maximun ink coverage of 240% resulting in a muddy looking black when printed.
When to use them
Standard black should be used for text and small areas to avoid overlapping colors. Rich black should be used for larger areas to get a darker black color.