Start with chocolate frosting or dark-colored icing as a base
Add black gel food coloring a little at a time
Mix thoroughly after each addition
Let the icing sit for 15 to 30 minutes to deepen the color
Add more black gel food coloring if needed
Use a tiny amount of blue or purple gel coloring to help neutralize brown tones
Avoid liquid food coloring if possible
Use a dark cocoa powder base for a richer black shade
Make the icing slightly ahead of time for best color development
Store covered until ready to use