Preheat the oven to 350 degrees. Coat an 8-inch square baking pan with cooking spray.
Place the graham crackers into a large ziplock bag and seal. Smash the crackers into fine crumbs using a mallet. Combine the graham cracker crumbs, melted butter, sugar, and lemon zest together in a bowl. Using your fingers, mix until graham cracker crumbs are moist and well combined. Pour the mixture into the prepared baking pan and press the mixture firmly into the pan. Bake the crust for 10 minutes or until set. Remove from the oven and let it cool. Do not turn off the oven.
Meanwhile, whisk the egg yolks, condensed milk, lemon juice, and lemon zest together in a bowl until well combined and slightly thickened. Gently fold in 1 cup of the blueberries. Pour the lemon blueberry filling over the crust and spread it evenly with a spatula. Sprinkle the remaining 1/4 cup of blueberries over the mixture in the bare spots.
Place the pan into the oven and bake for 15 minutes or until filling is just set. Cool to room temperature and then place it into the refrigerator for at least one hour before serving. Once chilled, cut into bars and serve. Enjoy!
Side Note: Bars will keep in the refrigerator for 3-4 days.