With the rack in the middle position, preheat the oven to 350°F (180°C). Butter the sides of an 8-inch (20 cm) springform pan and line the bottom with parchment paper.
In a bowl, combine the flour, cornmeal, baking powder and salt.
In another bowl, cream the butter and sugar with an electric mixer for 5 minutes or until the mixture turns pale. Add the egg and mix well. Add the ricotta and lemon zest. Stir in the dry ingredients. Transfer the batter to the prepared pan, then place the blueberries on top, spreading them out evenly.
In a bowl, combine the dry ingredients. Add the butter and mix with your fingertips until just moistened and crumbly. Press the mixture together with your hands, then roughly crumble it up, letting it fall in pieces over the blueberries.
Bake for 1 hour or until a toothpick inserted into the centre of the cake comes out clean. Let cool completely on a wire rack. Unmould.