If you have access to a bramble of blackberry bushes, guard it with your life. The price of blackberries makes it more than reasonable to pick your own, besides which nothing is quite so satisfying as picking your own berries. Perhaps it’s knowing where the food came from and taking part in the ancient practice of foraging. If you must use frozen berries, you should measure them while they’re still frozen, because they shrivel as they thaw. Also, it’s important to thaw and drain frozen berries before placing them in the pie shell, otherwise the pie will be watery. Use potato starch in this pie if you can. It gives the filling a clear, jewel-like color, and it has less flavor than cornstarch.
Whisk the potato starch and sugar together and set aside. Put 1 cup of the berries in a small saucepan and mash them with a fork. Cook and stir over medium heat for about 7 minutes, until the berries begin to break down and release their juice. Stir the starch mixture into the berries and cook and stir until thick and bubbling and the sugar is dissolved, about 7 minutes. The mixture should coat a spoon and a line drawn along the spoon will stay clear. Cool until lukewarm.
Spread the remaining berries in the crust, then pour the cooked mixture over the berries and gently stir until evenly distributed.
Chill for 3 to 4 hours or overnight, until the filling is set. Serve garnished with a dollop of whipped cream.