Cube the unsalted butter into 1/2-inch cubes and place into the freezer to chill for 10 minutes.
In a large mixing bowl (or in a food processor), combine the dry ingredients: flour, sugar, salt, baking soda and baking powder. If using a food processor, add the cold butter and pulse for about 30 seconds, until the butter is cut into small and medium-sized crumbles. You can also use a pastry blender or two large forks to cut the butter into the flour.
In a separate bowl, combine the wet ingredients: sour cream, heavy cream, eggs and vanilla. Whisk the ingredients together for a few minutes, until the eggs are well beaten.
Add about 1 cup of the butter and flour mixture into the wet ingredients and whisk until smooth. Add the remaining flour and using a spoon, mix until a soft dough forms and all the dry ingredients are incorporated. Do not over mix the dough or add more flour! The dough should be slightly sticky and have bits of butter visible.
Add the dried and fresh blueberries next and gently mix them into the dough without crushing the berries. Use your hands to pull apart the dough to get the berries incorporated.
Line your work surface with a few sheets of plastic wrap and turn the scone dough out into the center. Gently shape the dough into a ball, then gently flatten it into a disk, about 1 ½ to 2 inches tall and 9 inches across. Wrap the disk well with the plastic wrap and let the dough chill for at least 1 hour, or overnight if making ahead of time.
Preheat the oven to 350F/177C and line a large baking sheet with parchment paper or a silicone mat. Unwrap the scone dough and sprinkle the top with large sugar crystals. Using a sharp chef’s knife, portion the disk into 8 equal sized pieces. Transfer the scones onto the prepared baking sheet with the help of a small, off-set spatula, spacing the scones a few inches apart.
Bake the blueberry scones in the preheated oven for 30 to 35 minutes, until they’re golden brown on top. The scones will be soft and fragile straight out of the oven. Let them rest on the baking sheet for 15 minutes, then use a spatula to transfer them onto a wire rack to cool further. Before storing the scones, allow them to cool completely, then store in a ziplock bag or airtight container.