Slice the bread on the diagonal to create eight 3/4 thick pieces, heels removed. Arrange the bread slices in a 10X15 baking dish. To make the batter, whip together the eggs, milk, baking powder and vanilla in a medium bowl. Slowly pour the mixture over the bread, turning each slice to coat completely. Cover the dish with plastic wrap and refrigerate for at 1 hour, but preferably overnight. Preheat the oven to 425 degrees. Coat another 10X15 baking dish with cooking oil spray. Sprinkle the blueberries over the bottom of the pan. Mix together the sugar, cinnamon and cornstarch and pour evenly over the blueberries. 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.