Step 1: Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper.
Step 2: In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Step 3: Add the cold, cubed butter to the dry ingredients. Use a pastry cutter or your fingers to work the butter in until the mixture looks like coarse crumbs.
Step 4: Gently fold the blackberries into the flour and butter mixture, being careful not to break them.
Step 5: In a small bowl, whisk together the heavy cream, egg, and vanilla extract.
Step 6: Pour the wet ingredients into the dry ingredients and stir with a fork until a dough starts to form. Do not overmix.
Step 7: Turn the dough onto a floured surface and gently pat into an 8-inch disc. Cut into 8 wedges.
Step 8: Place the wedges on the prepared baking sheet. Brush the tops with heavy cream and sprinkle with coarse sugar.
Step 9: Bake for 18-22 minutes, or until the scones are golden brown on top and bottom.