Shred the zucchini on a large box grater. Transfer to a fine mesh strainer and drain while preparing the batter. (SEE TIPS)
Preheat oven to 350°F. Lightly grease a 13x9-inch baking pan. Set aside.
In a medium bowl, whisk together the flour, baking powder, baking soda and salt. Set aside.
In a measuring cup combine 1/2 cup buttermilk, lemon zest, lemon juice and vanilla. Stir to combine and set aside.
In a large mixing bowl beat together the butter and sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Add 1/3 of the flour mixture to the butter and eggs. Beat on medium until blended. Add half the buttermilk and mix until combined. Repeat alternating with half the remaining flour and then all the remaining buttermilk. Add all remaining flour and blend on medium until incorporated. Add the zucchini and blend until incorporated. Scrape the bottom and sides of the bowl with a spatula, then blend again.
Pour the thick batter into the prepared pan, smoothing the top with an offset spatula or spoon. Scatter the blueberries over the top slightly pressing the berries into the batter. Bake for 45 to 55 minutes or until the top is golden brown and a toothpick inserted in the center comes out clean. Moist crumbs are okay but no wet batter. Cool completely on a wire rack.