Preheat oven to 350⁰. Butter or grease with shortening a 9-inch square cake pan, dust with flour and set aside.
For the cake:
Sift together the flour, baking powder, and salt onto the sheet of wax paper and set aside.
In a medium bowl beat the butter until light and fluffy with an electric mixer on high speed. Gradually add the sugar and continue beating until the mixture is very light and fluffy. Change the mixer to speed to medium and beat in the egg.
Change the mixer speed to low. Beat in the sifted dry ingredients and the mil alternately, beginning and ending with the dry ingredients and beating only until combined.
Use a spatula to spread the batter in the prepared pan and smooth the top. Scatter the blueberries in an even layer over the batter. Squeeze the lemon juice over the blueberries.
For the crumble:
Combine the sugar, flour and cinnamon in a small bowl. Stir in the melted butter until the mixture forms the crumbs. Scatter the crumbs in an even layer over the blueberries.
Bake until the berries burst somewhat and the crumbs are golden, 1 hour and 5 minutes to 1 hour and 10 minutes. Serve warm or at room temperature, cut into 9 squares.