If you have large black areas on any pieces that you need printed, use Rich Black instead of Black.
Rich black is an ink mixture of solid black (100% K) with additional Cyan, Magenta and Yellow ink values. This results in a darker tone than black ink alone. If you print black ink alone, the resulting black may not be as dark as you might like.
To create Rich Black make your color values:
60% C, 40% M, 40% Y, 100% K