6 tablespoons unsalted butter, frozen, divided
1 ½ cups blueberries, frozen
½ cup low-fat buttermilk
1 cup fat-free Greek yogurt
2 cups all-purpose flour
½ cup granulated sugar
1 tablespoon baking powder
¼ teaspoon baking soda
½ teaspoon table salt
1 teaspoon lemon zest
Turbinado sugar or coarse sugar
Adjust oven rack to middle position and heat to 425°F. Line baking sheet with parchment paper and set aside.
Grate 4 tablespoons of frozen butter on a large-hole grater. Place grated butter in freezer until needed. Place blueberries in freezer until needed.
Whisk together milk and Greek yogurt in medium bowl; refrigerate until needed.
Whisk flour, sugar, baking powder, baking soda, salt, and lemon zest in medium bowl.
Add frozen butter to flour mixture and toss with fingers until thoroughly coated.
Add milk mixture to flour mixture and fold with spatula until just combined. Transfer dough to a well-floured work surface. Dust dough with flour and knead dough 8 to 10 times, until it forms a rough ball, adding flour as needed to prevent sticking.
Roll dough into a rough 12-inch square. Fold dough into thirds (like a letter). Lift ends of dough and fold into thirds again to form a 4-inch square. Transfer dough to plate lightly dusted with flour and chill in freezer 5 minutes.
Remove dough from freezer and transfer to a floured surface. Roll into an approximate 12-inch square again.
Sprinkle blueberries evenly over surface of dough and lightly press them into dough.
Using a scraper, carefully release dough from surface. Roll to form a tight log, jelly roll style. Lay dough seam-side down and press into a 12×4-inch rectangle. Using a sharp knife, cut log crosswise into 4 equal sections. Cut each rectangle diagonally to form 2 triangles. Transfer scones to parchment-lined baking sheet.
Melt remaining 2 tablespoons of butter and lightly brush tops of scones. Lightly sprinkle with turbinado or coarse sugar. Bake until tops and bottoms are golden brown, 20-25 minutes. Transfer to wire rack and cool before serving.