Add the blackberries, arrowroot, maple syrup, sugar, lemon zest, and salt to a large pot. Cook over medium heat, stirring frequently until the blackberry mixture begins to thicken (about 5-7 minutes). Remove from heat and let cool.
Roll out the remaining dough and either use a pizza cutter to create a lattice design, or cook cutters to create shapes. You can also use this piece of pie crust to create a double crust.
Preheat the oven to 375 degrees F. Once the blackberry mixture is cooled, pour into the baked crust. Top with cut out shapes, or the double crust. If doing the double crust, be sure to poke some slits in the top layer so it doesn’t bubble up and pop.
If using the egg wash, gently whisk together the egg with a tbsp of water. Brush the mixture over the prepared crust and the edges. You can sprinkle with turbinado sugar or raw sugar.
Transfer to the oven and bake for 40 minutes or until the crust is golden all over. If the pie seems to be browning quickly, you can also cover with foil.
Let the pie cool completely before slicing and serving. Store in the fridge.