Preheat the oven to 400°F, and line two baking sheets with parchment paper.
Cook the blackberries, apple, flour, cinnamon and 6 tablespoons of sugar in a heavy saucepan over medium heat, stirring often, until the mixture just boils and is thickened, about 5 minutes. Take care not to break up the berries too much while stirring. Transfer to a shallow bowl to cool.
Place a heaping tablespoon of fruit filling in the center of the empanada disc. Moisten the edges of the dough with milk and fold in half, pressing the edges to seal. Transfer to a lined baking sheet and press the tines of a fork around the edges or make a decorative repulgue [video]. Arrange the empanadas 1 inch apart on the baking sheet. Repeat the process with the remaining discs and filling.
Brush the empanadas with milk and sprinkle them with the remaining 2 tablespoons of sugar. Bake until empanadas are golden, approximately 15 minutes. Remove them from the baking sheets to cool.