Blue Red Velvet Cupcakes

Our answer

Nigella's Red Velvet Cupcakes (from Kitchen and on the Nigella website) get their maroon-red colour from a combination of cocoa powder and red food colouring. We have not heard of the cakes turning a different colour before though very occasionally we have seen slight discolouration in carrot cake and cakes containing acidic fruits such as raspberries due to the reaction of the bicarbonate of soda (baking soda) with pigments in the fruits. Usually this is because the bicarbonate of soda has not been mixed evenly into the batter.

However in this case it seems unlikely to be the primary cause and we suspect that you may have a baking powder which contains aluminium. This can react with acid to turn some baked good blue, though it is also quite unusual for this recipe as the bicarbonate of soda is alkaline and should to an extent balance the acidity of the buttermilk. We suggest checking your baking powder to see if it does contain aluminium. Also if you are making your own buttermilk then you should use only 1 (15ml) tablespoon of acid (lemon juice or vinegar) for 250ml (1 cup) milk.