In a small bowl, whisk together flour, baking powder and salt and set aside. In a large bowl, beat butter and sugar until light and fluffy. Add eggs, vanilla and milk. Mix in lemon zest. Add flour mixture and mix just until combine. Stir in blueberries.
Spread batter evenly into prepared pan. Sprinkle with crumb topping. Bake at 350º for 45 minutes or until a toothpick inserted into the center comes out clean. About 210 degrees with an instant read thermometer.