Preheat the oven to 375°F. Spray a 9 x 9-inch baking dish with baking spray. Set aside.
Wash and dry the blueberries.
Using an electric mixer, cream together ¼ cup softened butter with ¾ cup granulated sugar, lemon juice and vanilla extract. Add the egg beating until combined.
Sift together 2 cups self-rising flour with ⅛ tsp nutmeg.
Add the sifted flour to the creamed mixture alternately with the buttermilk beginning and ending with flour. Beat on medium just until fully combined. The batter will be thick.
Toss the blueberries with the remaining 1 Tbsp of flour. Fold the flour coated blueberries and lemon zest into the batter by hand. Spread the batter into the baking pan.
In a separate bowl, mix together the dry ingredients for the crumb topping, Use a pastry cutter to cut the cold butter into the dry ingredients until it's pea sized. .
Sprinkle the batter with the crumb topping. Bake for 30-35 minutes or until a toothpick inserted into the center comes back clean.
Cool completely before cutting into squares.