Puree the blueberries with a blender. Remove 1 tablespoon of the sugar, stir into the blueberries with the lemon juice and put the mixture aside.
Put the buttermilk with the vanilla extract, milk and the remaining sugar in a blender and mix until it foams. Pour the thick blueberry juice evenly into two large glasses and carefully pour the milk over them. Serve now! You can use more or less sugar according to taste.