Start with chocolate buttercream or dark chocolate frosting as the base
Add black gel food coloring a little at a time
Mix thoroughly after each addition
Let the icing sit for 30 to 60 minutes so the color deepens
Add more black gel coloring if needed
Use a tiny amount of blue or purple gel coloring to help neutralize brown tones
Avoid liquid food coloring
Use black cocoa powder for a darker base if desired
Keep the icing covered until ready to use
Stir again before piping or spreading