Preheat the oven to 425°F and line a baking sheet with parchment paper.
Add sugar and lemon zest to a medium bowl. Massage together until the sugar is pale yellow and smells of lemon.
Add the gluten-free flour, sugar, baking powder, xanthan gum if using, salt, and whisk to combine. (Or use a food processor)
Add the cold butter and using a pastry cutter or your hands, cut in the butter until it is the size of small peas. If you're using a food processor pulse a few times.
In a small bowl, whisk together the eggs, heavy cream, and vanilla extract. Add the wet ingredients to dry ingredients and stir until you have a shaggy dough. If you're using a food processor pulse just until the dough begins to come together. It should still look a little crumbly.
Transfer the dough to a lightly floured board and gently fold in the blueberries by hand, being careful not to crush them too much.
Turn the dough out onto the prepared baking sheet and gently pat it into a disk about 1 to 1½ inches thick. Cut into 6 triangles and use a spatula to separate them slightly on the baking sheet.
Brush the tops with a little heavy cream and sprinkle with extra sugar, if desired.
Bake for 14 to 17 minutes, or until the scones are lightly golden around the edges and set in the center.
Let the scones cool completely on the pan.
To make the glaze, whisk together the powdered sugar, lemon juice, lemon zest, and salt until smooth. The glaze should be thick but still easy to drizzle. Add a little more lemon juice if needed.
Drizzle the glaze over the cooled scones and let it set before serving.