Combine flour, sugar, vanilla sugar, and baking powder in a large mug. Add milk and oil; mix batter well with a fork or a small whisk. Carefully fold in blueberries so they are completely covered with batter.
Microwave on high until cake has risen by 50%, 30 seconds to 2 minutes, depending on strength of your microwave. Allow to cool in the mug for a few minutes. Dust with confectioners' sugar.