Defrost the blueberries. In the meantime, whisk the fat and 250 g sugar until creamy with the whisk of the hand mixer. Stir in eggs and milk bit by bit. Mix 250 g flour and baking powder and stir into the fat egg mass.
Cut the dough in half. Chop 100 g chocolate and melt over a warm water bath. Puree blueberries. Stir the chocolate into one half of the dough. First mix 2 tbsp. flour, then pureed blueberries into the second half of the dough.
Grease a box mould (approx. 30 cm long; 2 litres capacity) and sprinkle with breadcrumbs. First fill in dark, then light dough. Sort blueberries, wash, grate dry and, except for a few for decoration, spread on the dough.
Bake in a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer) for approx. 1 1/4 hours (stick test). Cover with foil after about half of the baking time. Remove the cake from the oven and let it cool down on a cake rack for about 30 minutes.
In the meantime beat the egg whites with salt until stiff. Let 100 g sugar trickle in. Finely chop the remaining chocolate and fold it into the meringue, except for about 1 tbsp. Remove the cake from the mould. Spread the beaten egg whites loosely on the top of the cake and sprinkle with the rest of the chocolate.
Bake again in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 8-10 minutes. Take the cake out of the oven and let it cool down completely on a cake rack. Arrange the cake on a plate and decorate with blueberries.