Start with a chocolate frosting base if possible
Add black gel food coloring a little at a time
Mix thoroughly after each addition
Use black cocoa powder for a darker base if needed
Add a small amount of blue or green gel coloring if the frosting looks brown or gray
Let the frosting sit for 30 to 60 minutes so the color deepens
Add more black gel coloring if needed
Avoid adding too much liquid food coloring
Use a white frosting base only if you want a true black color with more coloring
Store the frosting covered until ready to use