Preheat the oven to 375°F. Butter a 9-inch pan and sprinkle with a tablespoon of the sugar. Shake it around a bit to even out.
Pour the blueberries in a single layer into the dish. Set aside.
In a blender, add the remaining sugar, buttermilk, eggs, almond flour, GF flour, scraped vanilla bean or extract, lemon zest, lemon juice and salt. Blend until completely smooth, about 30 seconds. Pour the batter over the blueberries and gently tap to remove any air bubbles.
Bake for about 45 minutes, or until golden brown around the edges and an inserted toothpick comes out clean.
Allow the Clafoutis to cool just slightly, then sprinkle with powdered sugar through a sieve and serve.