Preheat the oven to 400F degrees.
Take the pie dough out of the fridge. Let it warm up for 10-15 minutes.
On a very well floured surface using a floured rolling pin, roll out 1 disc of pie dough into a circle about 12 inches (30 cm) in diameter.
Roll the dough up onto your rolling pin, then transfer the dough to a 9-inch pie plate.
Spoon the blackberry pie filling into the pie dough using a slotted spoon. Do not add the liquid from the bottom of your bowl.
Roll out the second disc of pie dough on your floured surface. If making a double crust: Transfer the dough to the top of the pie. Trim excess crust from the edges, fold over the edges, and crimp the edges. Use a sharp knife to cut an X-shape in the center. If making a lattice-topped pie: After rolling out the second disc, cut it into strips about ½ - ¾ inch thick using a pizza knife or sharp knife. Place 4-5 strips vertically on top of the pie, then weave 4-5 strips through horizontally, alternating between going over & under. Cut the excess off the edges, then fold the edges over and crimp them with your fingers.
In a small bowl whisk together the egg and milk for the egg wash. Brush the egg wash on top of the pie.
Bake in the preheated oven for 20 minutes at 400F, then turn the oven down to 375F and continue baking for 30 minutes. If the pie starts to brown but it's still early in the bake time, cover with tinfoil and continue baking.
Cool the pie fully, and let to rest for at least 3 hours before slicing into it.