I'd try talking her into fondant so it can be peeled off and the guest wont have black teeth and tounges. Otherwise I'd use chefmaster liqua gel to color the BC. It takes very little to get a true black and has no bitter taste nor do you need to let it set overnight to intensify.
You can also roll it out between 2 sheets of parchment paper. It won't stick as long as you release the parchment every once in awhile on both sides (just keep flipping it over and rolling from both sides).