Preheat your oven to 325F/160 C and line a baking sheet with parchment paper. To a bowl, add the almond flour, coconut flour, ¼ cup powdered erythritol, baking powder, and salt. Whisk to incorporate.
Cut the cold butter into small pieces and add to the bowl. Using your fingers, pinch the butter into the dry ingredients until the mixture resembles coarse bread crumbs. Chop the blackberries and stir them into the mixture.
In a small bowl, whisk the egg, heavy cream, almond extract, and vanilla. Add to the previous bowl and stir gently until combined. The dough needs to hold together barely, so add ½ – 1 tbsp of heavy cream if it is crumbly.
Don’t overwork the dough; you need the butter to stay cold in the dough. Transfer it to a clean surface and shape it into a rough circle. Press until 1 ½ inch thick (3 cm) and cut into 8 wedges.
Using a spatula, transfer the dough pieces to the lined baking sheet. Bake for 20-25 minutes until golden and risen. Remove from the oven and let cool completely.
To a small bowl, add ¼ cup powdered erythritol and 1 tsp of lemon juice. Whisk until a thick glaze forms. If the glaze is paste-like, add ½ tsp or 1 tsp of lemon juice until desired consistency.
Drizzle the glaze over the scones. Serve immediately or store until later.