Preheat oven to 350°F (175°C). Line a 9-inch (23 cm) square cake pan with parchment paper. Set aside for later.
In a large mixing bowl, combine sugar and lemon zest. Rub together with fingers or a spatula until fragrant. Add the melted butter, Greek yogurt (or sour cream), eggs, and vanilla extract. Beat with a mixer until smooth and slightly fluffy, about 2–3 minutes.
Add the baking powder, baking soda, salt, and cornmeal. Mix until combined. Gradually add the all-purpose flour, mixing just until incorporated. Do not overmix.
Spread about half of the batter into the prepared pan and smooth the top. Sprinkle with about half of the blueberries. Repeat with remaining batter and blueberries.
Bake for 40–45 minutes, until the cake pulls away from the sides and a toothpick inserted into the center comes out clean.
Cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely. Slice and enjoy!
