For the filling: Adjust oven rack to middle position and preheat oven to 350°F (175ºC). In a large bowl, whisk sugar, cornstarch, lemon zest, cinnamon, and salt, to combine. Add 6 cups of the blueberries and the lemon juice and toss to combine; set aside.

For the topping: In a large bowl, whisk together flour, ⅔ cup sugar, cornmeal, salt, baking powder, baking soda, and lemon zest in a large bowl. In a small bowl, whisk 8 tablespoons melted butter, sour cream, and egg until smooth. Create a well in the dry ingredients. Gradually add sour cream mixture to the flour mixture, stirring until just combined.

Brush the remaining 1 tablespoon butter over the bottom and sides of a 10-inch cast iron skillet or 9- by 13- inch glass baking dish, to coat. Add the blueberry mixture to the skillet. Using a large spoon, dollop the cornbread topping over blueberries. Sprinkle with remaining 1 tablespoon sugar.

Bake until the cornbread batter spreads into an even layer and is just beginning to set, about 10 minutes. Using oven mitts, carefully remove the skillet from the oven and top the cornbread with the remaining ½ cup blueberries, gently pressing the blueberries into the topping. Return to the oven and continue to bake until the blueberry filling is bubbling, the cornbread topping is golden-brown, and a knife inserted into the center of the cornbread layer comes out clean, 40 to 50 minutes. If needed, cover with foil to prevent excess browning towards the end of baking.

Let cool at room temperature for 15 minutes before serving. Serve with vanilla ice cream or whipped cream.