Preheat oven to 375°. Place blackberries in a lightly greased 8-inch square baking dish; sprinkle with lemon juice. Stir together egg, sugar, and flour in a medium bowl until mixture resembles coarse meal. Sprinkle over fruit. Drizzle 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, if desired. Garnish with fresh mint sprig, if desired.
For a neat presentation, bake for the same amount of time in 6 (8-oz.) ramekins on an aluminum foil-lined baking sheet.