Display Blacks Accurately in Illustrator
If your design incorporates blacks, then make sure to adjust your “Appearance of Black” settings in the Illustrator Preferences. Set the on screen appearance to “Display All Blacks Accurately”. This will help you more accurately adjust dark color values.
These are some of the kinds of black available.
The deepest darkest black possible is (C:100, M:100, Y:100, K:100)