Black is one of the paste or gel food colors that does impart a bitter taste to foods. You are unlikely to notice it in small quantities, but the more you use, the more likely you will taste the bitterness.
It is also hard to get foods black. In general you get some variation of gray or purple, and must be happy with creating the impression of black. You can create a strong impression, but it’s best not to look at your food with too critical an eye. An awful lot of people make a dark brown chocolate cupcake with a very dark cocoa and use that as their “black” cupcake, but, of course, it’s not black either.
Often, pairing your blackish creation with other contrasting colors will make it look much more black. The lines on the wings of Wilton’s bat cookie are made with violet buttercream piped onto the black buttercream on the wings. Taken together, the lines look blue and the wings look black, where the wings might otherwise look a little purple.
It is unlikely you would ever use enough black food color in a cupcake batter to turn it bitter – cupcakes are awfully sweet. It’s also unlikely you will ever produce a very black cupcake. You’ll be in much better shape though, if your batter starts out white, not yellow. Add a little food color to start and see how dark it turns your batter. Add a bit more if you like, but at some point you’ll have to draw the line.
We do not like to use too much black food color in anything – if it turns your tongue black, it’s way too much. We prefer to use black as an accent color. There’s nothing wrong with having, say, Halloween treats orange and white, decorated with black candies or hints of black frosting here and there.