Mix Dry Ingredients: In a large bowl, combine flour, granulated sugar, baking powder, baking soda, and salt.
Add Butter: Cut cold butter into the flour mixture using a fork or your hands until it forms a coarse, crumbly texture with no large butter pieces.
Add Buttermilk: Pour in buttermilk and stir with a spatula until a thick dough forms and the dry ingredients are fully incorporated.
Top the Blackberries: Drop spoonfuls of dough over the baked blackberries and gently pat down to cover.
Egg Wash: Whisk the egg and brush over the dough. Sprinkle with coarse sugar if desired for a crispy top.