Preheat the oven to 375°F (190°C). Lightly coat an 8×8-inch baking pan with cooking spray or line it with parchment paper.

Add the flour, granulated sugar, brown sugar, baking powder, cinnamon, allspice, and salt to a food processor. Pulse briefly until combined.

Add the cold cubed butter and pulse until the mixture forms small, coarse crumbs.

Add the egg, lemon zest, and vanilla extract. Pulse just until incorporated. Avoid overmixing.

Divide the crumb mixture approximately in half. Transfer one half to the prepared baking pan and gently press it into an even layer. The mixture will still look slightly crumbly but should hold together when pressed.

In a separate bowl, gently toss the fresh blueberries with the granulated sugar, lemon juice, and cornstarch until evenly coated.

Spread the blueberry filling evenly over the prepared crust.

Scatter the remaining crumb mixture over the blueberries, covering the filling evenly without pressing it down.

Bake the Blueberry Crumb Bars for 30–40 minutes, or until the crumb topping is lightly golden and the blueberry filling is bubbling around the edges.

Allow the bars to cool completely in the pan before slicing. Serve as they are or with a scoop of vanilla ice cream.