Use a dark chocolate buttercream or black cocoa frosting as the base
Add black gel food coloring a little at a time
Mix thoroughly after each addition
Let the frosting sit for 30 to 60 minutes to deepen the color
Add more black gel coloring if needed
Use a small amount of purple or blue gel coloring to neutralize any brown tint
Start with room-temperature frosting for easier coloring
Avoid liquid food coloring
Use black cocoa powder for a darker base if making chocolate frosting
Chill the frosting briefly if it becomes too soft while mixing
Store covered until ready to use