Prehead oven to 375 degrees.
Lightly grease a 9x13 inch pan.
Make the Crumb. In a medium bowl, whisk together the flour, sugar, baking powder and salt. Use a fork to blend in the butter and then the egg (the dough will be crumbly).
Press half of the dough into the prepared pan.
Make the Filling. In a separate large bowl, whisk together the sugar, cornstarch and lemon juice. Gently fold in the blueberries to coat evenly with the sugar mixture.
Spread the blueberry mixture evenly over the crust, drizzling any excess juice in the bowl over the filling.
Crumble the remaining dough evenly over the blueberry layer.
Bake for 45 to 50 minutes or until the top is light golden brown.
Cool completely before cutting.
