Preheat the oven to 350°F (175°C). Thoroughly grease the bottom of a 9x13-inch baking pan with non-stick cooking spray.
In a medium mixing bowl, combine the sugar cookie mix, butter, flour, and 1 egg. Mix until a cohesive dough forms. Press this dough uniformly into the base of the prepared pan.
Evenly distribute the blueberries over the prepared crust.
In a separate medium mixing bowl, combine the eggs, flour, sugar, heavy whipping cream, lemon zest, and salt. Whisk these ingredients together until the mixture is smooth. Pour this custard mixture over the layer of blueberries.
Place the pan in the preheated oven and bake for 50-60 minutes. The custard should be slightly jiggly in the center with the edges set. Once baked, remove from the oven and allow the bars to cool to room temperature before serving.
For optimal freshness, store the bars in a refrigerator.