Preheat oven 180°C fan forced. Line 12 muffin holes with paper cases.
Set 12 small Blackberries aside. Crush the remaining blackberries on a plate with a fork. Combine the food colour and vinegar in a small bowl, stir until smooth.
Cream the butter and sugar with hand beaters until light and creamy. Add purple vinegar mixture, beat until well combined. Add cocoa, beat until smooth. Add eggs one at a time, beating well. Stir in half the flour then half the buttermilk. Repeat with remaining flour and buttermilk. Fold in the crushed blackberries. Divide between 12 paper cases (about 1/4 cup per cake).
Bake for 20-25 minutes, or until cooked when a skewer inserted into centre comes out clean. Stand in pan for 5 minutes. Cool completely on a wire rack.
For the icing, beat cream cheese and butter with a hand mixer until combined. Add icing sugar and beat on low speed until smooth. Add purple colour, beat until evenly distributed.
Pipe onto cakes using a 13mm star nozzle. Top with reserved blackberries. Serve.