Preheat oven to 375°. Grease a 9×13 inch pan, set aside.
In a large bowl, whisk together both sugars, flour salt and baking powder. Using your hands, fork or pastry cutter, cut in the butter until it resembles a coarse meal. (I like to use my hands.) Mix in the egg until fully combined. Press half of dough into the prepared baking pan and bake for 7 minutes.
While the crust is baking, stir together the blueberries, lemon, sugar and cornstarch. Once crust is done, spread blueberry mixture over the warm crust and crumble the remaining dough over the blueberries. No need to make this absolutely perfect, it's looks nice when some blueberries show through. Place back into oven and bake for another 35-45 minutes or until top starts to turn slightly golden. Once done, allow them to cool for 20 minutes before cutting.
Bars will stay fresh for up to 5 days stored in an airtight container at room temperature or up to 7 days stored in the fridge. They will get soft after day 1, but this doesn't affect the flavor at all.