Prepare the Oven and Baking Sheet: Preheat the oven to 400°F and line a baking sheet with parchment paper.
Combine the Dry Ingredients: Add the flour, granulated sugar, baking powder, baking soda, and salt to a large bowl. Whisk until the dry ingredients are evenly combined.
Work in the Butter: Grate the very cold butter directly over the flour mixture. Toss the grated butter through the flour to coat it, then use a pastry cutter to cut the butter into the dry ingredients. A food processor can also be used for this step.
Mix the Wet Ingredients: In a separate bowl, beat the egg together with the heavy cream and vanilla extract until combined.
Form the Dough: Pour the egg mixture into the flour mixture and stir gently with a fork just until a shaggy dough begins to form. The dough should have the loose, biscuit-like character described in the source rather than being heavily worked.
Add the Blueberries: Fold the completely thawed wild blueberries into the dough toward the end of mixing, distributing them evenly without overworking the dough.
Shape the Dough: Transfer the shaggy dough onto the parchment-lined baking sheet. Use the parchment to help lift and press the dough together, kneading it only a couple of times until it can be formed into an approximately 8-inch disc. If the dough becomes warm, place it in the freezer for a few minutes before continuing. Cut the Scones: Using a sharp knife or bench scraper, cut the 8-inch dough disc into 8 wedges. Separate the wedges and arrange them with space between each one on the baking sheet.
Brush and Sugar the Tops: Lightly brush the tops of the scones with extra heavy cream, then sprinkle them with 2–3 tablespoons of coarse sanding sugar.
Chill Before Baking: Refrigerate the prepared scones for 15 minutes so the butter becomes cold again before baking.
Bake the Scones: Bake at 400°F for 22–25 minutes, or until the tops are golden brown.
Prepare the Blueberry Glaze: While the baked scones cool, whisk the powdered sugar with enough reserved blueberry juice to create a smooth glaze. Stir in the vanilla extract and pinch of salt. If the glaze needs additional thinning, add more blueberry juice or some cream.
Glaze the Scones: Once the scones have cooled, drizzle the blueberry glaze over the tops. Allow the glaze to dry and set for about 1 hour if desired.