Roll out one pie crust into a 12-inch circle, lay it in the pie plate with edges hanging over, cover, and chill for 30 minutes.
Roll out the second crust on parchment and cut into 1-inch strips. Chill those too.
Preheat your oven to 400°F.
Whisk an egg with a splash of water for the egg wash.
Toss blackberries with sugar, tapioca flour, salt, and lemon juice until coated.
Pour the filling into the pie crust and dot the top with butter.
Lay 4 strips of dough across the pie, brush with egg wash, then weave 4 more strips the other way.
Trim the edges, crimp, brush the whole top with egg wash, and sprinkle with sugar.
Bake on a sheet for 40–45 minutes until golden and bubbly. Cool on a rack for 4 hours.
Let me know if you want an optional shortcut version without the lattice too!