Preheat the oven to 375°F. In a large bowl, whisk the flour, ½ cup sugar, and ¼ teaspoon salt. Add the cold butter cubes and work them in with your fingers or a pastry blender until the mixture looks like coarse crumbs with some pea-sized pieces. Reserve ¾ cup of the mixture for the topping.

In a separate bowl, gently toss the blueberries with the remaining ¼ cup sugar, cornstarch, lemon juice, and ⅛ teaspoon salt until evenly coated.

Press the larger portion of the crumble mixture firmly and evenly into an 8x8-inch parchment-lined pan.

Spread the blueberry filling over the crust, then scatter the reserved crumble on top, squeezing some into small clumps. Bake 40–45 minutes until the filling is bubbling thickly and the topping is deep golden.

Cool the bars in the pan for at least 2 hours, then lift them out, slice into 9 squares, and serve.