Preheat your oven to 350°F. Grease an 8x8-inch or similar-sized baking dish.
In a large bowl, whisk together the flour, sugar, baking powder, and salt. Add the milk, melted butter, and vanilla extract, stirring just until combined.
Pour the batter into the prepared dish. Evenly distribute the blackberries over the top, drizzle the lemon juice, and sprinkle a light dusting of extra sugar over the berries.
Bake for about 50 to 60 minutes, or until the edges are deeply golden brown and the batter is set. Let cool slightly before serving. Enjoy warm, with a scoop of vanilla ice cream if desired.