Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan, or line the bottom with parchment paper and grease the sides.

In a medium bowl, whisk together 2 1/2 cups of flour, baking powder, baking soda, and salt. Set aside.

In a large bowl using a stand mixer or hand mixer, beat the softened butter, granulated sugar, and the zest of two lemons on medium-high speed for 3-4 minutes until light, pale, and fluffy.

Scrape down the sides of the bowl. Add the vegetable oil and beat until combined. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract and 2 tablespoons of fresh lemon juice. The mixture may look curdled; this is normal.

With the mixer on low speed, add the flour mixture in three parts, alternating with the buttermilk in two parts, beginning and ending with the flour mixture. Mix only until just combined; do not overmix.

In a small bowl, gently toss the blueberries with the remaining 1 tablespoon of flour. Using a spatula, gently fold the coated blueberries into the batter.

Pour the batter into the prepared pan and smooth the top. Bake for 35-45 minutes, or until a wooden skewer inserted into the center comes out with a few moist crumbs attached.

Let the cake cool in the pan on a wire rack for 15-20 minutes. Then, carefully invert it onto the rack to cool completely.

Once the cake is cool, prepare the glaze. In a small bowl, whisk together the powdered sugar and 2 tablespoons of lemon juice until smooth. Add more juice if you prefer a thinner glaze. Drizzle the glaze over the top of the cooled cake.