1.Adjust oven rack to middle position and heat oven to 425 degrees. Score and remove half of wrapper from each stick of frozen butter. Grate butter and place grated butter in freezer until needed. Melt 2 tablespoons of remaining ungrated butter and set aside. Save remaining 6 tablespoons butter for another use. Place blueberries in freezer until needed.
2.Whisk together milk and sour cream in medium bowl; refrigerate until needed. Whisk flour, 1/2 cup 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.
3.Add milk mixture to flour mixture; fold with spatula until just combined. With rubber spatula, transfer dough to liberally floured work surface. Dust surface of dough with flour; with floured hands, knead dough 6 to 8 times, until it just holds together in ragged ball, adding flour as needed to prevent sticking.
4.Roll dough into approximate 12-inch square. Fold dough into thirds like a business letter, using bench scraper or metal spatula to release dough if it sticks to countertop. Lift short ends of dough and fold into thirds again to form approximate 4-inch square. Transfer dough to plate lightly dusted with flour and chill in freezer 5 minutes.
5.Transfer dough to floured work surface and roll into approximate 12-inch square again. Sprinkle blueberries evenly over surface of dough, then press down so they are slightly embedded in dough. Using bench scraper or thin metal spatula, loosen dough from work surface. Roll dough, pressing to form tight log. Lay seam-side down and press log into 12 by 4-inch rectangle. Using sharp, floured knife, cut rectangle crosswise into 4 equal rectangles. Cut each rectangle diagonally to form 2 triangles and transfer to parchment-lined baking sheet.
6.Brush tops with melted butter and sprinkle with remaining tablespoon sugar. Bake until tops and bottoms are golden brown, 18 to 25 minutes. Transfer to wire rack and let cool 10 minutes before serving.
7.To Make Ahead:.
8.After placing the scones on the baking sheet, either refrigerate them overnight or freeze. When ready to bake, for refrigerated scones, heat oven to 425 degrees and follow directions in step 6. For frozen scones, heat oven to 375 degrees, follow directions in step 6, and extend cooking time to 25 to 30 minutes.