Preheat oven to 325 degrees. Add one tablespoon to 9-inch cast-iron skillet and spread all over the bottom and the edges. Set aside.
Place remainder of the butter in bowl of a stand mixer. Add sugar and then cream together with a paddle attachment for 2-3 minutes. Add eggs one at a time. Add vanilla and sour cream and beat together until smooth. Add flour and salt and mix just until combined. Fold in blackberries.
Pour batter into prepared skillet. Spread into one even layer. Sprinkle with turbinado sugar all over the top. Bake in preheated oven until cake is golden brown on top and cooked through, about 1 hour. Remove from oven and let sit for 1 hour before cutting.
Serve pound cake slightly warm or room temperature.