Preheat oven to 350°F. Lightly grease an 8x8 baking pan.
In a large bowl, gently stir together all filling ingredients. Transfer to the prepared pan.
Topping:
In a medium bowl, combine sugar, flour, baking powder, and salt. Add the vanilla extract and melted butter and stir until fully combined. The mixture will resemble a cross between a crumb topping and a sugar cookie dough.
Press pieces of the topping together with your fingers to form flat irregular shaped pieces. Place the pieces of topping all over the top of the blackberries. Bake for 30-35 minutes, until the berries are bubbling and the topping is beginning to turn light golden brown. Place the pan on a wire rack to cool for about 15 minutes; the juices will thicken up a bit as it cools.
Notes
*I prefer this recipe with fresh blackberries because they hold their shape better after baking. If you'd like to use frozen blackberries, keep them frozen and increase the flour in the filling to ¼ cup.Topping adapted from Pinch of Yum.