Slice the French bread diagonally into 10-14 slices, each one about 1/2 or 3/4 inch thick.** Arrange in a single layer in a 9x13 inch dish.In a medium bowl, whisk together eggs, milk, baking powder, and vanilla. Pour slowly over the bread, making sure to get all the bread wet. Turn the slices over and saturate them. Cover completely and chill overnight, or at least a couple hours.When you are ready to bake, preheat the oven to 425 degrees. Grease a new 9x13 inch pan.Combine blueberries, sugar, cornstarch, and cinnamon in the greased pan. Stir until combined, then spread it out evenly.Place the bread wettest side down on top of the blueberries in a single layer. Brush with the melted butter.Bake in the preheated oven for 24-26 minutes, or until golden brown.Sprinkle with powdered sugar and serve immediately!