Preheat the oven to 450°F. Toss part of the blueberries with sugar. This step enhances sweetness and flavor.
Combine flour, baking powder, and remaining sugar in a bowl. Whisk well to distribute everything evenly. This ensures proper rising during baking.
Pour in the buttermilk and stir gently. Mix until just combined. Fold in the sugared blueberries carefully.
Pour melted butter into the baking dish. Spread the dough evenly over the butter. Score the top into 9 squares and add remaining berries.
Bake for about 30–32 minutes until golden. Let the biscuits cool completely. Mix glaze ingredients and drizzle over cooled biscuits.