Preheat your oven to 425 degrees Fahrenheit. This hot temperature is essential for achieving the quick baking time and creating a beautiful golden-brown exterior on your scones.
Whisk together the low-fat milk and egg until the mixture becomes frothy. Set this aside, as you'll use part of it to bind the dough and reserve the remainder for glazing the scones.
In a large bowl, combine the flour, baking powder, and salt. Blend these dry ingredients together thoroughly to ensure the leavening agent is evenly distributed.
Add the cold butter cubes to the dry ingredient mixture. Using your hands or a pastry blender, work the butter into the flour until the mixture resembles coarse breadcrumbs with no lumps larger than a pea. This crucial step creates the desired texture in your finished scones.
Stir in the white sugar and frozen blackberries, tossing gently until the berries are evenly coated with flour. This flour coating helps keep the blackberries suspended throughout the dough during baking.
Slowly pour in the egg mixture while stirring gently until the dough just comes together. You may not need all of the egg mixture, so reserve any remaining amount for the glaze. Avoid overmixing, as this can result in tough scones.
Turn the dough out onto a clean counter and knead it no more than 12 times (count carefully). Pat the dough into a round disc approximately 1/2 inch thick.
Cut the dough into 8 equal wedges, as you would slice a pizza. This ensures even baking and consistent portion sizes.
Transfer the wedges onto an ungreased baking sheet, parchment paper, or Silpat. Any of these three options will work equally well for baking your scones.
Using a pastry brush, lightly brush the remaining egg mixture over the top of each scone wedge. This glaze will create a beautiful golden color and add richness to the baked scones.
Place the baking sheet in the preheated oven and bake for 10 to 15 minutes, or until the tops are golden brown. Begin checking at the 10-minute mark, as scones bake quickly at this high temperature.
Transfer the baked scones to a wire rack to cool. Using a wire rack allows air to circulate underneath, preventing the bottoms from becoming soggy.
Serve the scones while they are still warm, accompanied by butter, jam, or clotted cream for an authentic and delicious treat.