Preheat the oven to 425 degrees F.
Put the blackberries, sugar, nutmeg, lemon juice, zest, salt, flour, and tapioca into a bowl and stir and fold until everything is evenly coated. Using the back of a wooden spoon, mash gently to make a textured filling.
Roll out the bottom dough and place it in your pie pan.
Pour the filling into the crust and set it aside.
Roll out the remaining dough, lay it over the fruit, trim, crimp, and cut 5 to 6 vents on top, or cut strips and weave a lattice top.
Brush the crust with the egg white mixture.
Bake at 425 degrees F on the middle rack of the oven until the crust is golden brown, about 15 to 20 minutes. Reduce the heat to 350 degrees F; bake until golden brown and bubbling, about 30 to 40 minutes more (mine baked an additional 80 minutes!). When there are about 10 minutes of bake time left, open the oven, pull the pie out, and quickly and evenly sprinkle the top of the pie with sugar (I did this around the 20 minute mark, so it baked about another hour with the sugar on top, but didn’t burn–in the future I would likely sprinkle the sugar on at the start so it can better stick to the egg wash).
Let the pie cool before serving so the filling can set up.