1Spray 4 1/2-quart slow cooker with baking spray with flour.
2Pour blackberries into slow cooker. Sprinkle berries with flour and 3 tablespoons of the sugar; stir gently to coat. In small bowl, mix remaining sugar, melted butter, cornbread mix and cinnamon. Spread dough on top of blackberries.
3Cover; cook on High heat setting 2 to 3 hours or until dough on top is set and blackberries are saucy. To serve, spoon hot cake from slow cooker, and top with ice cream or whipped cream.