Preheat oven to 375°F. Spray 9 x 5-inch loaf pan with cooking spray.
Stir together topping ingredients in a small bowl. Set aside.
In a medium bowl, use an electric mixer to combine the sugar and butter. Then, mix in the milk and the egg until smooth.
In a separate bowl, whisk together the flour, baking powder and salt. Stir into the wet mixture just until combined. Gently fold in the blackberries and lemon zest.
Pour the batter into the prepared pan and sprinkle with the topping. Bake until topping is deep golden and has formed a thick crust (50 to 60 minutes). Cool in pan 5 minutes, then transfer to a wire rack to continue cooling.
Keep an eye on the topping while baking. If it begins to darken/burn, cover with foil for remainder of baking.