Preheat oven to 375 degrees. Butter a 9 x 13 inch baking dish. Set aside.

Make the filling. In a medium mixing bowl, toss the blueberries, granulated sugar, cornstarch, lemon juice, and salt until evenly coated.

Pour the filling mixture into the prepared baking dish. Set aside while you make the topping.

Make the topping. In a medium mixing bowl, combine the flour, 1/2 cup sugar, brown sugar, baking powder, and salt. Add the butter and using a pastry blender cut into mixture until the mixture looks like a coarse meal.

Pour the boiling water into the flour mixture and stir, just until the mixture comes together, and is mixed through.

Drop spoonfuls of the dough topping over the blueberries. Sprinkle coarse sugar evenly over the top of the dough.

Place the baking dish on a cookie sheet, in case cobbler bubbles over and drips, and bake for about 40 minutes or until the top is golden and baked through and blueberries are tender.