To make black frosting add black food coloring to chocolate frosting instead of white frosting. You'll need far less coloring that way. If you add enough black coloring to white frosting to get it really black it will probably taste quite bitter, so I always recommend starting with chocolate frosting. If you only have white frosting, then add some cocoa powder (preferably Dutch Cocoa powder which is darker than natural cocoa powder) to the frosting before adding the black coloring. If the frosting gets too thick, add a few drops or more of water or milk to thin it out.