Cook blackberries, apple, semolina flour, cinnamon & 6 tablespoons sugar in a saucepan over medium heat, stirring frequently, until mixture just boils and is thickened, about 8 minutes. Allow to cool.
Put oven racks in upper and lower thirds of oven and preheat oven to 375°F Line baking sheets with parchment.
Roll out half of dough 1/8 inch thick on a lightly floured surface with a lightly floured rolling pin & cut into 6 (5-inch) squares, reserving scraps. Place a heaping teaspoon of fruit filling into center of 1 square. Moisten edges of square with milk and fold into a triangle, pressing edges to seal. Transfer a lined baking sheet and press tines of fork around edges of triangle. Repeat with remaining dough and filling, re-rolling all of the scraps together once to make 12 triangles total.
Brush triangles with milk and sprinkle with remaining 2 tablespoons of sugar. Bake, switching position of pans halfway through baking, until pies are golden, about 40 minutes total. Transfer pies to racks to cool.