Mix the flours, baking powder, sugar and blueberries together in a mixing bowl. Put the remaining ingredients in a jug and fork together. Add to the dry ingredients and mix briefly with a fork.
Divide the mixture evenly among paper cake cases arranged in a 12-hole deep muffin tin. Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 18–20 minutes until well risen and the tops are cracked. Leave to cool in the tin for 15 minutes.
Make the frosting. Sift the icing sugar into a bowl and gradually mix in enough lemon juice to make a thin spoonable icing. Take the muffins out of the tin and drizzle icing from a spoon in random lines over the top. Leave to harden slightly and serve the muffins while still warm.