Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish with butter or cooking spray.
In a large bowl, combine the blackberries, sliced peaches, 1/2 cup sugar, lemon juice, and vanilla extract. Toss gently to coat the fruit. Pour this fruit mixture into the prepared baking dish, spreading it evenly.
In another bowl, mix the flour, remaining 1/2 cup sugar, baking powder, salt, and cinnamon.
Pour in the milk and melted butter to the dry ingredients, mixing until just combined; do not overmix. The batter should be somewhat lumpy.
Gently pour the batter over the fruit mixture in the baking dish. Do not stir; the batter will rise during baking.
Bake in the preheated oven for about 40-45 minutes, or until the top is golden brown and a toothpick inserted into the batter comes out clean.
Cool for about 10 minutes before serving warm.
Notes
Serve warm with vanilla ice cream and a sprig of mint for added freshness.