1. Preheat the oven to 350°F (175°C). Line a 9×9-inch pan with parchment paper, leaving some overhang on the sides so the bars can be lifted out easily.

Prepare the Crust

1. In a bowl, stir together the graham cracker crumbs and sugar. Add the melted butter and mix with a fork until evenly moistened and resembling wet sand.
2. Press the mixture firmly into the bottom of the prepared pan. Place in the refrigerator to chill while preparing the filling.

Make the Crumb Topping

1. In a bowl, stir together the flour, granulated sugar, and brown sugar.
2. Add the melted butter and mix until coarse crumbs form. Place in the refrigerator.

Make the Cheesecake Filling

1. Beat the softened cream cheese, sugar, and vanilla until smooth.
2. Add the sour cream and cornstarch, mixing until combined.
3. Add the eggs and mix just until incorporated. Do not overmix.

Prepare the Blueberry Filling

1. Lightly mash about 3–4 tablespoons of the blueberries with a fork in a bowl.
2. Add the remaining blueberries, lemon juice, sugar, and cornstarch. Stir until evenly combined.

Assemble and Bake

1. Spread the cheesecake filling evenly over the chilled crust.
2. Spoon the blueberry mixture on top and gently spread it out.
3. Sprinkle the crumb topping evenly over the blueberries.
4. Bake for about 45–55 minutes, or until the center is set. Start checking after 35 minutes and tent with foil if the top is browning too quickly. The bars will puff up slightly while baking, then settle as they cool.
5. Let the bars cool in the pan for about 30 minutes, then lift them out using the parchment overhang.
6. Cool completely, then refrigerate for at least a few hours before slicing.