Start with a dark base icing, such as chocolate buttercream or dark cocoa frosting
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 in color
Add more black gel food coloring if needed
Use a tiny amount of purple or blue gel coloring to help neutralize brown tones
Avoid adding too much liquid food coloring
If the icing looks gray, add more black coloring and mix again
If the icing is too thin, add more powdered sugar or a little more buttercream
Chill the icing briefly if needed before final use
Use a piping bag or spread immediately once the desired color is reached