1. Butter and flour an 8-cup pound cake pan. Set aside. Preheat oven to 375F.
2. Place butter in mixing bowl. Add sugar and beat until light and fluffy. Add eggs, one at a time, beating after each. Add lemon zest and vanilla; Beat well. Add sour cream; Beat well. Add flour and salt and mix gently. Fold in blackberries. Pour batter into prepared pan.
3. Bake 1 hour or until toothpick comes out clean. Let pan sit on a damp towel (the steam helps loosen the cake) 5-10 minutes. Invert cake and place top up on cake stand.