The colour of the frosting comes from the blueberries so no need to add food colours.
To flavor the butter cream I made a thick concentrated blueberry paste. I personally think a cream cheese frosting may have suited the flavours better. The lemon cupcake was a regular cupcake recipe with the addition of lemon rind and juice.
To make the blueberry paste:
This made three tablespoon of paste with is enough for twelve cupcakes or 2 cups of butter icing
¾ cup fresh blueberries
1 tbsp sugar
2 tbsp water
Put these in a pan and set to simmer. Mash and then strain. Put back into the pan.
Next, mix ¾ teaspoon cornflour with 1 tablespoon hot water. Add this to the pan. Boil once and re-strain. Refrigerate. Add to your favourite frosting at the end and mix well.