Heat the oven to 375°F and butter a 3-quart baking dish.
Toss the blueberries with 2/3 cup sugar, cornstarch, lemon juice and 1/4 teaspoon salt, then spread in the dish.
Whisk the flour, 1/3 cup sugar, baking powder, lemon zest and 1/2 teaspoon salt.
Cut in the butter until the mixture is crumbly, then fold in the buttermilk just until a dough forms.
Drop the dough over the berries in nine small mounds.
Bake for 38 to 42 minutes until the biscuits are browned and blueberry juice bubbles through the center gaps; cool 25 minutes.