Preheat oven to 400 degrees F. Line a baking sheet with parchment paper.
In a medium bowl, mix flour, sugar, baking powder, baking soda and salt. Grate butter into flour mixture using the large holes of a box grater (mixture should resemble coarse meal.) This step sounds difficult but it's easy- your hands just get a bit buttery. It takes about a couple minutes to grate the entire cube of frozen butter.
Add in the sour cream, vanilla and egg and stir until dough pulls from the side of the bowl and there are no crumbs on the bottom. Dough will be somewhat firm and sticky, but still manageable.
Add in the fresh blackberries. Now some of the berries will burst and the dough will turn a bit purple- this is okay! Stir gently until most of the berries are incorporated. Flour a counter top and use your hands to shape the dough into a circle that's about 1" thick. If some of the berries escape just squish them back in- it's a very technical process. ;) Your hands will get purple and messy- just dig deep and let your inner child enjoy this part!
Once you've got the dough in a decent circular shape, use a pizza cutter to cut the circle in half through the center, then again on the opposite side. Make 2 more cuts across the circle diagonally. You should have made 4 cuts and have 8 pieces.
Place each piece on a greased cookie sheet and sprinkle with the remaining 2 TBSP sugar, if desired.
Bake until golden, about 15 to 17 minutes in a 400 degree F oven. Cool for 5 minutes.
To make vanilla glaze, whisk ingredients together and drizzle over warm scones.