Preheat oven to 350 degrees. Spray a 9 by 5 inch loaf pan with cooking spray.
In the bowl of an electric mixer, preferably fitted with the paddle attachment, cream the butter and 1 cup sugar on medium speed until light and fluffy, about 5 minutes. Turn the speed to low and add the eggs, one at a time, mixing well after each addition. Add the lemon zest and mix well.
In a mixing bowl, whisk together the flour, baking powder, baking soda, and salt. In another small mixing bowl, stir together the lemon juice, buttermilk, and vanilla.
With the mixer on low, add the flour and buttermilk mixtures to the butter, starting with half the flour, then the buttermilk, and finally, the rest of the flour. Scrape the sides and bottom of the mixing bowl well to make sure everything is well combined. Fold in the blueberries. Pour the batter into the prepared pan and smooth the top with a spatula.
In a small bowl, combine the flour, sugar, salt, and lemon zest for the topping. Add the butter and work it in with your fingertips until crumbly. Spread the topping over the top of the cake.
Bake the cake for 40-50 minutes or until a toothpick inserted into the center comes out clean. Let cool in the pan, then transfer to a serving platter.