Preheat oven to 375. Place 4 cups fresh blackberries in a lightly greased 8 inch square baking dish; sprinkle with 1 TBS. lemon juice. Stir together 1 large egg, 1 cup sugar, and 1 cup all-purpose flour in a medium bowl until mixture resembles coarse meal. Sprinkle over fruit. Drizzle 6 Tbsp. melted butter over topping. Bake at 375 for 35 minutes or until lightly browned and bubbly. Let stand 10 minutes. Serve warm with whipped cream and mint sprig.