Preheat the oven to 350°F. Grease a 9x13 baking dish or line it with parchment paper.
In a large bowl, stir together the melted butter, granulated sugar, brown sugar, and vanilla extract.
Add the flour, baking powder, salt, and cinnamon. Stir until the mixture forms soft crumbs.
Press about two-thirds of the crumble mixture firmly into the bottom of the prepared baking dish.
In a separate bowl, gently stir together the blueberries, sugar, cornstarch, lemon juice, lemon zest if using, vanilla extract, and salt.
Spread the blueberry filling evenly over the crust.
Sprinkle the remaining crumble mixture over the top.
Bake for 45 to 50 minutes if using fresh blueberries, or 50 to 55 minutes if using frozen blueberries. The topping should be lightly golden and the blueberry filling should be bubbling around the edges.
Cool completely before slicing. For cleaner slices, chill the bars for about an hour before cutting.
