I know this won't help now, but next time, try using a darker base color before adding the black AND letting it set for several hours before you use it.
Starting with a brown or combination of leftover colors is what many decorators do to get a dark black. I add cocoa powder to my icing that I am going to color black first, so that it needs less coloring and masks some of the bitter taste of the color with chocolate flavor.
Letting the icing sit in an airtight container for several hours or overnight before using will allow the color to deepen, so you have to use less to get the color you want.
The more coloring you add, the more time it takes to dry.