Preheat oven to 350°F. Thoroughly grease a 9x13 baking dish and set aside.
Prepare crumb topping: In a medium bowl combine flour, sugars, cinnamon, and salt. Add melted butter and stir until fully incorporated. Cover and refrigerate while you prepare the batter.
In a large bowl, combine flour, sugar, salt, baking powder, and baking soda.
In a separate bowl, combine eggs, buttermilk, milk, melted butter, lemon zest, and vanilla extract (the butter might clump from the chill of the cold ingredients- that's ok). Whisk wet ingredients until well combined. Add wet ingredients to the dry ingredients and use a rubber spatula to stir until combined. The batter will be lumpy; do not over mix.
Pour the batter into the greased baking dish. Sprinkle the blueberries over the top. Remove the crumb topping from the fridge and crumble over the top of the blueberries and batter. Bake for about 35-45 minutes, until puffed and light golden brown. Serve warm with maple syrup.