Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish.
In a mixing bowl, combine the blackberries, 1/2 cup of the sugar, lemon juice, and vanilla extract. Toss gently to coat the blackberries and set aside to marinate for about 10 minutes.
Spread the blackberry mixture evenly in the bottom of the greased baking dish.
In another bowl, whisk together the cake mix, remaining 1/2 cup sugar, milk, cinnamon, and salt until just combined. Do not overmix; lumps are okay for a rustic texture.
Pour the cake batter over the blackberries in the baking dish, spreading it evenly to cover the fruit.
Drizzle the melted butter over the top of the cake batter evenly. This will create a crispy top layer as it bakes.
Bake in the preheated oven for 45-50 minutes or until golden brown and a toothpick inserted into the cake portion comes out clean.
Let the cobbler cool slightly before serving.
Notes
Serve slices warm, topped with a scoop of vanilla ice cream or a dollop of whipped cream, and a sprinkle of fresh mint for a pop of color.