Place 175 g blueberries in a tall container and defrost. Put 300 g blueberries and 50 g sugar in a pot and bring to the boil at low heat. Mix starch with 3 tablespoons of water. Remove blueberries from the stove, stir in starch.
Bring the berries to the boil again for about 1 minute, pour into a bowl and allow to cool while stirring occasionally.
Puree the defrosted blueberries with lemon juice. Cream 250 g butter, 200 g sugar, salt and vanilla sugar with the whisk of the hand mixer. Stir in the eggs one by one. Mix flour and baking powder and stir in alternately with the milk.
Cut the dough in half. Mix one half with 125 g blueberry puree, the other half with the smoothie.
Pour light-coloured dough into a greased, flour-dusted Guglhupform (24 cm Ø; 3 litres). Spread blueberry compote on top. Add the blueberry dough and swirl with a fork. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for about 1 hour. Take out and let cool down.
Cream 50 g butter and icing sugar with the whisk of the hand mixer. Stir in cream cheese and whip for approx. 5 minutes. Stir in the rest of the blueberry puree. Chop chocolate separately and melt over a warm water bath.
Carefully turn the cake out of the tin, spread with dark chocolate and let it dry for about 10 minutes. Decorate with white chocolate, let dry again. Add blueberry cream.