Place your fresh blueberries in a single layer on a sheet tray and place in the freezer for at least 15 minutes, preferably longer. This will help prevent them from bursting when mixing into the biscuit dough.
In a large mixing bowl, whisk together the dry ingredients; flour, baking powder and baking soda, sugar, and salt. Add in cold butter that has been cut into cubes. Toss to coat with flour mixture.
Use a finger snapping motion to press the butter and flour together, a process known as "cutting in the butter".
You should end up with small pieces of butter incorporated into flour that has the texture of lumpy sand. Add in your blueberries and toss to coat in the flour.
Make a well in the center of the mixture and add the cold buttermilk. Using a rubber spatula or wooden spoon, gently bring everything together until you have a fairly uniform dough.
If there are a lot of dry spots, add one or two tablespoons more buttermilk and mix in. Dump the dough out onto a lightly floured surface.
Using your hands, gently bring it together into a rough square.
Pat or roll the dough into a rectangle at least ½ inch thick. If desired, add more blueberries to the top of the dough for beauty reasons. Using a 2 inch biscuit or cookie cutter, cut out your biscuits.
Gather your scraps as you go, gently re-form them together, and cut as many biscuits as you can. You should get 8 to 12 biscuits, depending on the size of your cutter.
Place your biscuits on a parchment paper lined small sheet pan. Put the sheet pan of biscuits in the freezer or refrigerator to chill while you preheat the oven to 425°F.
Remove biscuits from the freezer and brush the tops of the biscuits with an egg wash of one egg whisked with one tablespoon of water or milk. Bake in a 425° oven for 18-20 minutes, or until golden brown.
In a small bowl, whisk together the powdered sugar, cream, and lemon juice. If it is too thin, add more powdered sugar, a couple of tablespoons at a time. If too thick, add more lemon juice or cream.
Allow biscuits to cool for about 10 minutes, then drizzle generously with your glaze.