Rich black is an ink mixture of solid black, 100% K, with added CMY ink values. This CAUSES a darker tone than black ink alone. When you print black alone as 100% K, the resultant black might not be as dark as you may like.
100% K ON SCREEN
100% K AFTER PRINT
C 60 M 40 Y 40 K 100
This will provide a dark, deep, rich black.
C 100 M 100 Y 100 K 100