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