Put the flours in a bowl or food processor. Add the butter and rub in with the fingertips or process until the mixture resembles breadcrumbs. Add the cinnamon, oats and muscovado sugar and stir in or blend briefly until the mixture makes coarse crumbs and starts to cling together.
Tip about half the mixture into a greased 28 x 18 cm (11 x 7 inch) shallow baking tin and spread in an even layer. Pack down with the back of a spoon to make a firm base. Scatter with the blueberries and sprinkle with the remaining crumble mixture.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for about 50–60 minutes until the crumble is deep golden. Leave to cool in the tin, then sprinkle with vanilla sugar and cut into bars.