Heat oven to 350º. Spray an 8×8 inch baking dish with cooking spray and set aside.

In a medium bowl, combine the graham cracker crumbs, sugar and lemon zest. Stir in the melted butter until all the crumbs are moistened. Press firmly into the bottom of the prepared baking dish. Bake for 10 minutes, then cool to room temperature.

Meanwhile, whisk together the egg yolks and condensed milk in a medium bowl. Then whisk in the lemon juice and zest and stir until smooth. Gently fold in blueberries.

Pour the filling over the cooled crust and bake for 15 minutes, until just set.

Cool to room temperature, then refrigerate for at least an hour before cutting and serving. Bars are best served chilled and will keep in an airtight container in the refrigerator for 3-4 days.