Preheat oven to 375 degrees. Place blackberries in a lightly greased 8-inch square baking dish or 6 (8-ounce) ramekins.
Sprinkle berries with lemon juice. Stir together egg, sugar and flour until it resembles coarse meal. Sprinkle over berries. Drizzle with melted butter. Bake for 3 to 5 until lightly brown and bubbly. Let stand for 10 minutes. Serve warm. Top with whipped cream and fresh mint sprig if desired.