Preheat oven to 400°F.
To prepare shortcakes, lightly spoon flour into dry measuring cups; level with a knife. Combine flour and next 5 ingredients (through salt) in large bowl, stirring with a whisk. Cut in chilled butter with a pastry blender or 2 knives until the mixture resembles coarse meal. Add buttermilk and stir just until moist.
Turn dough out onto a lightly floured surface; knead lightly 4 times. Pat dough to a ½-inch thickness, cut with 3-inch biscuit cutter to form 8 dough rounds. Place dough rounds 2 inches apart on baking sheet coated with cooking spray. Combine water and egg white, stirring with whisk; brush over dough rounds. Sprinkle evenly with 2 teaspoons sugar. Bake at 400°F for 13 minutes or until golden. Remove from oven, and cool on a wire rack.
To prepare filling, combine 1 cup blueberries, ½ cup sugar, juice, and cornstarch in small saucepan. Bring to a boil, reduce heat, and simmer 5 minutes or until slightly thick. Place in a large bowl. Add remaining blueberries and blackberries, stirring to coat. Cover and chill.
Using a serrated knife, cut each shortcake in half horizontally. Spoon ½ cup blueberry mixture over bottom half of each shortcake. Top each serving with 2 tablespoons whipped topping and top half of shortcake.
Recipe courtesy of Cooking Light CookingLight.com