Preheat oven to 375 degrees. Butter a 9X13 inch baking dish. Set aside.
Make the Filling: In a medium mixing bowl, toss the blueberries, granulated sugar, ground tapioca, 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 granulated sugar, brown sugar, baking powder, and salt. Add the butter and using a pastry blender or 2 knives to 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 just mixed through.
Drop large spoonfuls of the dough topping over blueberries. Sprinkle coarse sugar evenly over top of dough.
Place the baking dish on a cookie sheet, in case the cobbler bubbles over and drips, and bake for about 40 minutes or until the topping is golden and baked through and the blueberries are tender.