Preheat oven to 400 degrees F. Line a baking sheet with a silicone baking mat or a piece of parchment paper In a large bowl, sift together the dry ingredients (flour, baking powder, salt, and sugar).Use a pastry blender to cut the butter into the flour mixture until the pieces of butter are no larger than the size of peas. Fold the blueberries into the flour mixture.
In a separate small bowl, combine the cream and the vanilla extract.
Pour the wet ingredients over the flour-blueberry mixture, then use a fork to very gently combine. Continue stirring until a shaggy dough forms.
Turn the scone dough out onto a well floured work surface and knead 2-3 times. Pat dough into a circle, then flatten using a rolling pin or your hands until about 1 1/4 inches thick. If there is too much flour on top, brush it off gently with a pastry brush. Cut into 8 even slices using a sharp knife or a bench scraper. Place the unbaked scones on the prepared baking sheet. For best results, chill for 15 minutes in the refrigerator before proceeding.
Brush the tops of the scones with a little heavy cream. Bake in a hot oven for 15 to 20 minutes until golden brown.
While the scones bake, make the glaze. Combine the glaze ingredients in a separate bowl. Add more water, if too thick, or more powdered sugar, if too thin. The glaze should have the texture of Elmers glue.
Remove the baked scones to a wire rack to fully cool. Once the scones reach room temperature, drizzle with the glaze. Let the glaze set before serving.