PREP: Preheat oven to 400 degrees with a rack in the upper third. Line a baking sheet with parchment paper.
MIX DRY INGREDIENTS/CUT IN BUTTER: In the bowl of a food processor fitted with a steel blade, measure in the flour, sugar, baking powder, and salt. Pulse a few times to mix. Add the pieces of butter and pulse carefully only until you have pea-sized and smaller pieces of butter.
TURN OUT FLOUR MIXTURE INTO A LARGE BOWL and gently stir in the blueberries. In a small bowl, whisk together the sour cream or yogurt and milk and tip that into the flour mixture. Using a table fork, stir until all the dry bits are incorporated, but the dough is still shaggy. (Takes a minute or two; be patient.) Knead a few times until the dough barely holds together, then turn the dough out onto a lightly floured counter or board. The dough needn't be uniform, smooth, or neat. Uneven edges are ok as this is a rather rustic scone.
PAT DOUGH with floured hands into a 1-inch-thick rectangle, approximately 6"x10". Brush lightly with some the egg mixture and sprinkle with a little granulated sugar. (You won't use all of the egg; cook the rest for the dog.) Using a bench scraper or a knife, cut into first 2, then 4, then 8 (or then 16 for small scones) rectangles. Place on prepared sheet pan as far apart as possible.
BAKE scones until golden, 18-22 minutes. Remove to a rack and cool for a minute or two before serving warm. Scones will keep for a day or two, well-covered. To reheat, split in half lengthwise, spread lightly with butter, and grill until toasty and hot in a stove top grill pan or skillet. To freeze baked scones, cool completely, wrap well and freeze for up to 3 weeks.