Preheat oven to 425 degrees Fahrenheit. Whisk the milk and the egg together until frothy and then set aside. In a large bowl, blend the flour, baking powder, and salt. Add the butter and blend the mixture with your hands until you have no lumps larger than a pea. Add the sugar and the blackberries and toss about until the blackberries are coated with the flour mixture. Add the egg mixture slowly until the dough comes together. Leave any remaining egg mixture to brush on as glaze.
Turn dough out onto a clean counter and knead it no more than 12 times. Pat dough into a round approximately 1/2 an inch thick, and cut into 8 wedges. Place on an ungreased baking sheet or a Silpat. Using a pastry brush, glaze wedges with remaining egg mixture. Bake for 10-15 minutes, or until golden. Cool on a wire rack.