Preheat the oven to 350°F. Lightly grease a pie pan and set aside.
To make the filling, add the blackberries, sugar, and lemon juice to a large saucepan and bring it to a simmer. Stir in the cornstarch and cook until thickened. Let cool to room temperature before assembling the pie.
While the filling is cooling, make the pie crust. In a medium bowl or the bowl of a stand mixer, mix the flour and salt. Grate the cold butter into the flour mixture, and cut in using a pastry blender or the paddle attachment, until the pieces of butter are no larger than pea sized. Add the buttermilk, and mix until the dough just starts to come together.
Turn out onto a floured surface and gently work into a ball, handling as little as possible. Shape the dough into a disc. Wrap in plastic wrap and refrigerate at least twenty minutes, or until ready to use.
To make the streusel, whisk the flour, sugar, and brown sugar together with a fork in a small bowl. Drizzle in the melted butter and whisk until the butter is blended in and the mixture is crumbly. Refrigerate until ready to use.
To assemble the pie, roll the dough out on a lightly floured surface to fit your pie pan. Gently press the dough into the pie pan and roll or pinch the edges to form the crust. Pour the filling into the pie crust, spreading evenly. Sprinkle the streusel over the top in an even layer.
Bake 55-60 minutes, or until the crust is golden brown, the streusel is browned, and the filling is bubbling and thick. Let cool completely before slicing and serving.
