In a mixing bowl, combine blackberries, sugar, lemon juice, cornstarch, and vanilla extract. Set aside.
In another bowl, mix gluten-free flour and salt. Cut in the cold butter until the mixture resembles coarse crumbs. Gradually add ice water, mixing until the dough comes together.
Roll out the dough and place it in a pie dish. Pour the blackberry mixture into the crust.
Cover with another layer of rolled-out dough or create a lattice top. Seal the edges.
Bake for 45 minutes or until the crust is golden brown and the filling is bubbly.