1. Heat oven to 400 degrees F. Sift together first three ingredients.
2. Cut in butter until the mixture looks like pebbles. Mix in water, 1 tablespoon at a time, until dough begins to come together.
3. Form into a ball, cover with a towel, and let rest 5 minutes.
4. On a lightly floured surface, roll into a 12-inch round. Gently place in a 9-inch pie pan; flute edges.
5. Lightly prick bottom of crust with a fork. Bake 8 minutes. Let cool on a wire rack.
6. Meanwhile, in a large bowl, combine lemon juice and sugar.
7. In another bowl, beat eggs and egg whites together. Add to lemon juice mixture and beat until mixture turns a bright lemon color. Add zest and butter.
8. Reduce oven temperature to 350 degrees F. Place blackberries in bottom of pie shell.
9. Pour lemon mixture over blackberries. Bake 30 minutes. Let cool completely on a wire rack.