Line 9" pie plate with pastry dough.
Position racks in oven at center and bottom. Preheat to 425°F.
Use a small grinder or food processor, grind tapioca for a minute. Set aside.
Place 3 cups blueberries in medium saucepan over medium heat. As they begin to warm, use potato masher several times to crush about half the berries to release juices. Continue cooking and stirring about 8 minutes until reduced to 1 1/2 cups. Set aside to cool slightly.
Place grated apple in clean kitchen towel and squeeze dry. Transfer apple to large bowl. Add cooked berries, remaining 3 cups uncooked berries, and remaining filling ingredients. Toss to evenly combine. Transfer to dough-lined pie plate.
Cover filling with top crust and seal edges. In a small bowl, whisk together egg and 1 teaspoon water. Brush egg-wash on top crust. Sprinkle with sugar. Line a baking sheet with foil or parchment paper and place pie in center.
Protect edges of crust from over-browning by wrapping with foil. To do this take a length of foil that is a bit longer than half pie circumference. Fold in half lengthwise and tear to cut in half. Fold each in half along the length. Join the two together to make one long strip. Wrap strip around edge of pie plate so it is perpendicular; it shouldn't be on top of crust. Join ends by rolling together and so foil fits snugly.
Place pie on baking sheet on bottom oven rack; bake 20 - 25 minutes. Pastry should be set and just beginning to brown.
Lower oven temperature to 375°F and transfer pie on baking sheet to center rack. Continue baking 20 minutes longer. Remove foil protecting edges. Continue baking 10 - 15 minutes longer until juices are bubbling throughout and crust is a deep golden brown.
Cool completely on a wire rack, about 4 hours, to allow filling to set.