Preheat oven to 400 degrees. Lightly spray 9-inch pie dish with nonstick cooking spray.
Position 1 unbaked crust into the bottom of the pie pan, trimming the edges if necessary.
Arrange blackberries into the bottom of the crust.
To make the custard filling, combine sugar, cornstarch, salt, eggs, vanilla, and buttermilk in your blender. Blend until smooth, about 45 seconds.
Pour custard over blackberries.
Top berries with remaining pie crust and crimp the edges to seal. Cut an “X” into the center of the top crust for steam to escape while baking.
In a small bowl, whisk together egg and water.
Brush the top crust with egg wash and sprinkle with sugar.
Lightly cover pie with foil and bake for 30 minutes.
Remove foil and bake for an additional 25-30 minutes or until crust is golden brown.
Cool for at least 20 minutes before slicing.
Notes
To the custard filling, feel free to add a pinch of nutmeg and/or cinnamon for extra warm flavor.I have not attempted using frozen blackberries in this recipe. I certainly think you could try it, but I do worry a bit about the amount of water that frozen berries tend to release. Custard pies can be finicky and turn watery/not set if the fruit releases too much liquid while cooking.