As we have noted elsewhere, black is tough. There are only so many food colors approved by our Food and Drug Administration, so black food color is made of some combination of blue, red, and yellow food colors. In the container it looks very black, but in real-life applications, it can look gray or purple-gray. The more food color you use, the darker your final product, but black food color can also impart a bitter flavor to foods, and there is some practical limit to how much you want to use and consume.
There are also spray-on black food colors, but to us, they also look gray/purple.
People have the most success with black frosting when they start with a dark chocolate frosting (made with the darkest cocoa you can find) and add reasonable amounts of black food color (don’t even ask us how to make black food color from the little vials of red, yellow, blue, and green coloring you find in every grocery store – it can’t be done!
The Valrhona cocoa is quite dark, and we’ve found it at quite a reasonable price. Bendorp’s is also dark, as is Droste’s. Hershey’s makes a Special Dark cocoa, but it is likely to be as hard to find as any of the fancier brands. Ghirardelli’s cocoa is not dark.
Once you’ve made your black frosting, do yourself a favor and only use contrasting colors for accents. If you use anything that is really black – like licorice – it will only proclaim how not black your frosting really is.
Here’s a chocolate frosting recipe to get you started.