Preheat the oven to 350°F.
In a large bowl, gently mix the blueberries with 1/2 cup granulated sugar, the brown sugar, cornstarch and lemon juice until berries are coated. Layer blueberries into the bottom of a greased 9x13-inch baking dish.
Whisk the flour, remaining sugar, baking powder, salt and cinnamon in another large bowl. Cut in the butter until mixture is mealy. Slowly add the buttermilk while stirring gently until combined. The batter will be sticky and look like cookie dough.
Scoop blobs of the batter onto the top of the berries, distributing as evenly as possible. Don’t worry if berries show through – this is supposed to be a rustic look.
Bake uncovered for 1 hour. Sprinkle coarse turbinado sugar across the top upon removing from oven (optional).
Serve warm with a scoop of ice cream.