Preheat the oven to 350 degrees F. Grease an 8 x 8-inch baking pan.
For the crumble, combine the flour and the sugar, add the melted butter, and mix until well blended. Set aside
In a large bowl combine the 1/4 cup butter, 3/4 cup sugar, and lemon zest, with an electric mixer, mix until well blended. Add the egg and mix in.
In another bowl mix the 1 cup flour, baking powder, baking soda, and salt together.
Add 1/3 of the flour mixture to the batter and mix in. Add 1/2 of the milk and mix in. Repeat the steps ending with the last 1/3 cup.
Fold in the blueberries. Spread the batter in the prepared baking pan. Sprinkle the crumble over the top of the batter evenly.
Place in the preheated oven and bake for 35 to 40 minutes or until a wooden pick comes out with a few moist crumbs.
Remove from the oven and place on the cooling rack until completely cooled.
Add the powdered sugar to a small bowl and add enough lemon juice to make a thin icing. Drizzle over the cooled coffee cake.