Sprinkle the frozen blueberries with flour and put them back in the freezer until you incorporate them into the mixture.
In a large mixing bowl, whisk together the flour, sugar, salt, and baking powder.
Work in the butter just until the mixture is unevenly crumbly; it's OK for some larger chunks of butter to remain unincorporated.
In a separate mixing bowl, whisk together the eggs, lemon extract, and heavy whipping cream.
Add the liquid ingredients to the dry ingredients and stir until all is moistened and hold together.
Gently incorporate the frozen blueberries, folding the dough 3-4 times.
Line a baking sheet with parchment. Sprinkle a bit of flour atop the parchment or pan.
Scrape the dough onto the floured parchment or pan, and make a circle. The circle should be about 3/4 inch thick.
Place the pan of scones in the freezer for 30-60 minutes, uncovered. (Chilling the scones relaxes the gluten in the flour, which makes the scones more tender and allows them to rise higher. It also chills the fat, which will make the scones a bit flakier.)
Preheat the oven to 425°F.
After chilling, using a knife or bench scraper that you have run under cold water and dried, slice each circle into 6-8 wedges.
Carefully pull the wedges away from the center to separate them just a bit; there should be about a 1/2 inch space between them, at their outer edges.
Brush chilled scones with heavy whipping cream being sure to cover all exposed edges. Sprinkle with turbinado sugar.
Bake the scones in the upper part of your oven for 18-23 minutes, or until they are a light golden brown. When you pull one away from the others, it should look baked all the way through. (If there is wet batter but the scones are golden brown, feel free to cover with foil and continue baking.)
Remove the scones from the oven, and cool briefly on the pan. Serve warm.