Preheat the oven to 350°F and line a 9 X 9 inch baking pan with parchment paper.

In a mixing bowl, combine the crumb topping ingredients: flour, light brown sugar, and cinnamon. Cut in the softened butter until it clumps together. Refrigerate while preparing the batter.

In another bowl, whisk together the flour, baking powder, and salt. Set aside.

Add lemon zest and granulated sugar to a stand mixer bowl. Rub zest with fingers to release oils.

Cream the lemon sugar and softened butter together until light and fluffy, about 3 minutes.

Gradually add the eggs, one at a time, mixing until combined. Scrape down the bowl.

On low speed, add half of the flour mixture and mix. Add sour cream and lemon juice; mix until smooth. Add the remaining flour mixture and mix until just combined; a few dry spots are okay.

Toss blueberries with a tablespoon of flour, then fold them into the batter gently.

Spread the batter in the prepared pan, sprinkle reserved blueberries on top, and scatter the crumb topping evenly.

Bake for 30 to 35 minutes, or until a toothpick comes out clean or with a few moist crumbs.

Cool on a wire rack before removing from the pan and slicing into squares.