Adjust oven rack to lower-middle position and preheat to 400 degrees.
Grind the tapioca in a spice grinder, or just cleaned coffee grinder, until it becomes a fine powder.
Whisk together sugar, ground tapioca and salt in a mixing bowl. Add blackberries, lemon zest, lemon juice and water and fold until 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 dough-lined pie plate. Dot with butter. Arrange strips in a tight lattice pattern. Trim excess on ends of dough off strips flush with the overhang of the bottom crust. Fold lattice ends and bottom crust under. Flute edges using your thumb and forefinger or crimp with a fork.
Brush lattice top with heavy cream and sprinkle with coarse sugar.
Place pie plate on a rimmed baking sheet. Bake in a preheated oven, until 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.