Blend the first four ingredients to a smooth batter. Grind the hazelnuts, but not too fine, and keep back about 1½ tablespoons. Stir the hazelnuts and blueberries into the cake mix and spoon into a loaf tin, 1 kg (2lbs) in capacity, lined with baking parchment. Make a depression down the centre with the back of the spoon. Mix together the reserved hazelnuts, sugar and cinnamon and sprinkle on the surface of the cake. Bake for about 45 minutes in a preheated oven at 180°C/350°F/gas mark 4, or until a skewer inserted in the middle comes out clean.
Remove from the oven, cool in the tin for 5 minutes, then lift out by holding the baking parchment, and allow to cool completely on a wire rack.