Preheat the oven to 375 degrees F. Butter the bottom and sides of a 9-by-13-inch glass or light-colored metal baking pan. Put a long piece of parchment paper in the bottom of the pan, letting the parchment extend up the two short sides of the pan and overhang slightly on both ends. (This will make it easy to remove the bars from the pan after they have baked.) Butter the parchment.
In the bowl of a stand mixer fitted with the paddle attachment combine 1 cup sugar, flour, and baking powder on low speed. Add salt and lemon zest and mix until combined. Add the butter, toss with a fork to coat with the flour mixture. Add egg and mix on medium-low speed until the texture resembles coarse cornmeal, with the butter pieces no larger than small peas. Pat half of dough into the prepared pan. Place reserved half aside.
In another small bowl, stir together the 1/2 cup sugar, cornstarch and lemon juice. Gently mix in the blueberries. Sprinkle the blueberry mixture evenly over the crust. Crumble remaining dough over the berry layer.
Bake in preheated oven for 45 minutes, or until top is slightly brown. Cool completely on wire rack then transfer to the refrigerator to chill before cutting into squares. Store in an airtight container in the refrigerator.