Adjust oven rack to lower-middle position and preheat to 400°F.
Grind the tapioca (3 tablespoons) in a spice grinder, or just a cleaned coffee grinder, until it becomes a fine powder.
Whisk together sugar (1 1/2 cups), ground tapioca, and salt (1/8 teaspoon) in a mixing bowl. Add blackberries (8 cups), lemon zest (2 teaspoons), lemon juice (2 tablespoons), and water (2 tablespoons), and fold until the mixture is evenly coated. Allow to set on the counter at room temperature for 15 minutes.
Roll one of the pie crusts, on a lightly floured counter, into a 12-inch circle. Fit into a standard 9-inch pie plate. Trim excess dough leaving a 1-inch overhang. Put back in the fridge while you prepare the second crust.
Roll out the second crust, on a lightly floured counter, into a 12-inch circle. Cut into 12-one inch strips with a knife or pastry wheel.
Spoon blackberry mixture into a dough-lined pie plate. Dot with butter. Arrange strips in a tight lattice pattern. Trim excess off ends of dough strips flush with the overhang of the bottom crust. Fold lattice ends and bottom crust under. Flute edges using your thumb and forefinger or crimps with a fork.
Brush lattice top with heavy cream (2 tablespoons) and sprinkle with coarse sugar (1-2 tablespoons).
Place the pie plate on a rimmed baking sheet. Bake in a preheated oven, until the crust is golden brown and the juices of the pie have slow bursting bubbles in the center of the filling, about 50-60 minutes. If crust begins to brown too quickly, cover loosely with aluminum foil.