To make the crumble:

• Preheat oven to 350°F. Line an 8x8" square baking pan with enough parchment paper that so it hangs over the sides. (You'll use it as handles to lift the bars out of the pan later.)
• Stir together flour, oats, ⅓ cup granulated sugar, brown sugar, baking powder and cinnamon in a bowl. Stir in vanilla and butter. Reserve ⅓ of mixture (about 1 cup) in a small bowl and refrigerate. Press remaining mixture into bottom of prepared baking pan. Bake for 10 minutes. Set aside to cool slightly.
  1¼ cups all-purpose flour, 1 cup rolled oats, ⅓ cup granulated sugar, ⅓ cup brown sugar,, 1 teaspoon baking powder, ½ teaspoon ground cinnamon, ½ teaspoon vanilla extract, 8 tablespoons unsalted butter,
• While the crust is baking, add blueberries, lemon zest, lemon juice, ¼ cup granulated sugar and cornstarch to a small saucepan. Heat over medium heat and stir together until sugar is melted. Continue to cook for another 3-5 minutes or until the blueberry juices are slightly thickened and coat the back of a spoon.
  3½ cups blueberries, zest of 1 lemon, 1 tablespoon fresh lemon juice, ¼ cup granulated sugar, 1 tablespoon cornstarch
• Pour warm blueberry mixture into crust. Sprinkle with remaining oatmeal mixture from refrigerator. Bake for 35-45 minutes or until the edges bubble and the top is golden brown. Cool completely.
• When ready to serve, lift bars from the pan using parchment paper. Place on a cutting board and cut into 16 pieces.