I was hunting for a black icing that also tasted good. I found a dark chocolate buttercream icing recipe on the Wilton website, and I also discovered that King Arthur makes a black cocoa (it’s much darker than the Hershey’s Special Dark cocoa). You might have to hunt the cake shops to find it, or you can order at http://www.kingarthurflour.com/shop/items/black-cocoa-16-oz
I suppose if you want a super dark chocolate icing that’s NOT black, you can just omit the black food color. But if you do want it black, you need to make it a day in advance for the color to deepen (as with all deep colored icings).