Preheat your oven to 375°F (190°C).
Grease the baking dish with butter.
In a bowl, mix blackberries with 1/4 cup of sugar.
Combine flour, remaining sugar, baking powder, and salt in another bowl.
Stir in milk, melted butter, and vanilla until smooth.
Pour batter into the baking dish.
Sprinkle blackberries evenly over the batter.
Bake for 45-50 minutes until golden brown.