"black coffee at its most basic is coffee at its most pure... / ... black coffee is just about presentation and sugar can be added to a cup of black coffee for extra sweetness. The only real rule of black coffee is that milk, cream, or any other type of whitener cannot be added. This is because it will change the color of the coffee, meaning that your cup of black coffee is no longer black, ..." Read more here:
Black Coffee - Coffee and Caffeine