Black Cake Tip. When it is hot and humid black fondant really does not behave well. Try a black crusting buttercream instead. You don't have to add as much black food colouring because buttercream darkens over time. The cake below started as a medium black and darkened overnight. You can take out as much liquid as you added black ( 1/4 teaspoon black food colouring take out 1/4 teaspoon liquid). Here is a little topsy turvy that I did this weekend finished in black buttercream.