Preheat oven to 350 F. Butter 13 x 9 baking dish. Place blueberries, sugar, cinnamon and cornstarch in dish. Stir to coat berries, spreading to cover bottom of dish evenly. Place bread slices, wettest side up, atop berry mixture, wedging slices in tightly and if necessary, cutting some slices to fit. (you may not use all of the bread). Brush tops of bread slices with melted butter. Bake in center of oven 20 to 25 minutes or until toast is golden and berries are bubbling. Let stand 5 minutes before serving. To serve, life toast onto plates, spooning blueberry sauce over top.