We get asked all the time about natural black color. What’s available is a mix of plant colors that tend to be green, blue or pink, and often can look brown or gray. Activated charcoal has become popular in recent years, but is not approved by the FDA as a food color.
A great alternative is black cocoa powder (dutch cocoa) to get a dark saturated black color.
Here is a simple recipe for naturally black colored icing. The icing does have a bit of a cocoa flavor.
1 cup powdered sugar
1 ½ Tbsp black cocoa (12 grams)
1 ½ Tbsp milk