This is because you will need to add a lot of colour to achieve the right shade, which is expensive and will also affect the consistency of the sugar paste.
To make black buttercream try adding cocoa powder, and then black paste food colour.
If you want to make black or dark royal icing, you will need to buy pure powder dust colour, rather than adding liquid, to keep the consistency stable.
This is mostly sold in industrial qualities, but the House of Sugar (www.houseofsugar.co.uk) in Wales supplies smaller quantities. Eddie Spence MBE, the royal icing guru who decorated the Queen's Diamond Jubilee Cake, uses House of Sugar.
When making coloured icing always make a bit more than you need and keep it to one side for emergencies
Matching the inside of the cake to the outside icing with works well with some novelty cakes, for example an orange pumpkin cake with a bright orange cake inside really looks good. I would not recommend making a black cake though!
Sue Haskell has been a bakery lecturer at Brooklands College, Weybridge for twenty years. She is well known in the baking industry, participating at exhibitions such as Cake International and Food and Drink Expo. She has helped with the competitions at the Alliance for Bakery Students and Trainees annual conference since 2000.
Brooklands College in Weybridge, Surrey, offers part time catering and sugarcraft IVQ courses, such as a Level Two in Professional Bakery and an ABC Sugarcraft Award Level One as well as one day courses including Design and Create an Easter Egg, Valentine's Cupcakes and Sugar Flowers for beginners.