Preheat oven to 375 degrees F. Spray 2 6-donut pans with cooking spray.
Sift flour with cornmeal, sugar, cornstarch, baking powder, baking soda, and salt in a large bowl.
In a separate bowl, whisk together blueberry kefir with melted butter, egg, lemon zest, and vanilla. Pour wet ingredients into the bowl with dry ingredients and whisk until combined. Fold in blueberries.
Fill donut pans halfway with batter. Bake donuts for 10-12 minutes or until a toothpick inserted in the center comes out clean. Let cool in the pan for 5 minutes. Transfer to a wire rack and cool completely.
Meanwhile, make the glaze. Simmer 1 cup blueberries with ¼ cup water in a medium saucepot over medium heat. Continue to cook for 10 minutes or until blueberries have softened and liquid has thickened. Strain blueberries, mashing them to extract as much juice as possible. Discard berries and reserve juice.
Beat together 2 tablespoons blueberry juice with 1 cup powdered sugar and 2 teaspoons elderflower syrup. Working with one donut at a time, dip the donut in the glaze. Alternatively, drizzle it on top with a spoon. Repeat with remaining donuts. Serve.