Whisk flour, sugar, lemon zest, baking powder, and salt together in a large bowl. Grate the frozen butter using a box grater. Add it to the flour mixture and combine with a pastry cutter, two forks, or your fingers until the mixture comes together in pea-sized (or a little smaller) crumbs. Place the bowl in the freezer before you continue.
Whisk 1/2 cup heavy cream, the egg, and vanilla extract together in a small bowl or a liquid measuring cup. Remove the flour mixture from the freezer. Drizzle the cream mixture over the flour mixture, add the blueberries, then gently mix together with a spatula or wooden spoon until everything appears moistened and the ingredients are just combined.
Lightly flour a work surface/counter. Pour the dough onto the counter and, with floured hands, work dough into a ball as best you can. Dough will be sticky. If it’s too sticky, add a little more flour. If it seems too dry, add 1–2 more Tablespoons heavy cream. Press into an 8-inch disc, about 1 inch thick. Using a sharp knife or bench scraper, cut the disc into 8 wedges.
Store leftover scones covered tightly at room temperature for 2 days or in the refrigerator for 5 days.