For the topping: In a small bowl, stir together sugar and lemon zest. Rub between your fingers; set aside. For the muffins: Preheat the oven to 350 degrees F. Line 12 muffin cups with muffin liners or spray with nonstick cooking spray. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt; set aside. In the bowl of a stand mixer, add the sugar and lemon zest. Rub the lemon zest and sugar together with your fingers. Attach the paddle attachment and beat the butter, sugar and lemon zest mixture together until light and fluffy, about two minutes. Add the ricotta cheese and beat until smooth. Beat in the egg, lemon juice, and vanilla extract. Using rubber spatula, fold in flour mixture until just combined (do not overmix.) (batter will be thick) Fill each muffin cup halfway with batter. Add three blackberries to each muffin cup. Top the muffins with remaining batter so blackberries are covered. Sprinkle lemon sugar topping evenly over muffins. Bake muffins for about 20-25 minutes, or until the tops are slightly golden brown. Cool muffins in muffin tin for 5 minutes before serving.