Mix sugar, baking powder, salt, shortening, buttermilk and 3/4 cup of flour to form a sticky dough. Spread remaining flour on a work surface and knead dough until most of the flour is incorporated and dough is manageable. Roll dough to about 1/4 inch thickness and cut into large pieces. Cover blackberry filling with dough, overlapping pieces, patchwork style.
Mix all ingredients and put into prepared pan.