Very, very few people want an all black. Yes! indeed it will make their teeth/mouth ugly
Those dark colors should be used very sparingly!
When you are finished decorating save all the **little**, tiny bits,& dabs of icing - like what you scrape out of the bag & the 'plug' of icing in the tip. Doesn't matter what color they are. Keep them in a jar in the fzr - adding to it each time you decorate.
When you need black, stir it up and add some black *super* coloring until you get a charcoal gray. You will be surprised how little it will take.
If at all possible make this at least the night before needing to use it - or early in the day so it has several hours to sit. The color will develop deeper over that time. Any unused can be returned to the fzr.