Preheat the oven to 350°F. Combine melted butter, sugar, lemon extract, lemon zest and flour in a large bowl. Stir until well combined and a crust dough forms.
Set aside 1 cup of the loosely packed dough to use for the crumble topping. Line an 8x8 square pan with parchment paper. Press the remaining dough into an even layer across the bottom of the prepared pan.
Bake the crust for 18 minutes. Note: the baked crust will go back in the oven after the filling is added.
In a clean large bowl, use a hand mixer to beat together cream cheese, 1/3 cup sugar, lemon juice, lemon zest, and egg until smooth and creamy.
Gently fold in 2 cups of fresh blueberries.
Spread the cream cheese mixture over the baked crust in an even layer. Sprinkle the reserved crumble dough over the top.
Top with the remaining 1/2 cup of fresh blueberries.
Bake for an additional 45-50 minutes, or until the top is golden brown.
Cool completely before lifting from the pan and cutting into bars.