Preheat oven to 425 degrees F. Lightly coat an 8×8 inch pan with cooking spray, line with parchment paper, then coat once more. Set aside.
In a large bowl, combine the all-purpose flour, whole wheat flour, granulated sugar, baking powder, cinnamon, and salt. Gently fold in blueberries and lemon zest.
In small, separate bowl, whisk together the milk, egg, and butter. Add to the flour and blueberry mixture. Stir by hand with a wooden spoon, just until combined. The batter will be very thick. Scrape into the prepared pan and spread into an even layer.
Prepare the crumble topping: In a small bowl, combine the flour, brown sugar, and cinnamon. With a fork (or your fingers), cut in the butter, combining until the mixture resembles coarse crumbs. Mix in the walnuts.
Sprinkle crumble mixture evenly over the top of the batter. Bake for 20 to 25 minutes, until a toothpick inserted in the center comes out clean. Serve warm or at room temperature.
