Slice the bread on the diagonal to create eight ¾-inch thick slices, heels removed. Arrange bread slices in 10x15-inch baking dish.
To make batter, whip together eggs, milk, baking powder, and vanilla in medium bowl. Slowly pour mixture over bread, turning over each slice to coat completely. Cover and refrigerate for at least 1 hour, but preferably overnight.
In the morning: Preheat oven to 425 degrees F. Coat another 10x15-inch baking dish with vegetable spray. Sprinkle blueberries over bottom of pan. Mix together sugar, cinnamon, and cornstarch and pour evenly over berries. Tightly wedge the bread slices over the blueberries, wettest side up. Brush the bread with melted butter. Bake the french toast in the center of the oven for 20 to 25 minutes, or until golden brown. To serve, place the toast berry side down on warmed plates. Stir the remaining berry mixture in the baking dish, then scoop over the toast. Sprinkle with powdered sugar.