In another small bowl, toss blueberries with 1 to 2 tablespoons sugar and the cornstarch to coat. Place berries in center of spray side of foil; top with granola and butter mixture. Bring up 2 sides of foil so edges meet. Seal edges, making tight 1/2-inch fold; fold again, allowing space on sides for heat circulation and expansion. Fold other sides to seal.