Thaw puff pastry at room temperature for at least 30 minutes. Preheat oven to 400°F. In a large bowl, toss blueberries with sugar and cornstarch. Spray a large baking sheet with nonstick cooking spray. Unfold pastry and place on baking sheet. Mound blueberries in center, leaving a 1-1/2-inch border. Drape pastry over blueberries; press pleats firmly together. Brush pastry with milk; sprinkle lightly with cinnamon sugar. Bake in the bottom third of oven until crust is golden, about 30 minutes; cool on a rack. Sprinkle pastry with confectioners sugar.