Adjust an oven rack to the middle position and preheat the oven to 350°F (180°C). Butter and flour a 12-cup bundt pan and set it aside. If using cold butter, microwave it for 15–20 seconds to soften but not melt. If the eggs are cold, place them in warm water for 5 minutes to bring them to room temperature.

In the bowl of a stand mixer, beat the butter and sugar together on medium speed until light and fluffy, about 2–3 minutes. Crack the eggs into a small bowl to check for freshness. Add them one at a time, beating well after each addition. The batter may look slightly curdled—this is normal. Mix in the lemon zest and lemon juice until combined.

Reduce the mixer speed to low and add the flour in three additions, folding gently with a spatula after each addition until fully incorporated. Do not overmix. If using fresh blueberries, toss them with 1 tablespoon of flour in a small bowl to help prevent sinking. If using frozen blueberries, do not thaw—add them directly. Gently fold the blueberries into the batter with a spatula.

Transfer the batter to the prepared bundt pan and smooth the top. Bake for 40–45 minutes or until a toothpick inserted in the center comes out clean. Let the cake cool in the pan for 10 minutes. Then, place a clean kitchen towel on the counter and firmly tap the pan on the towel to loosen the cake. Invert onto a wire rack and let it cool completely before glazing.

In a small bowl, whisk together the powdered sugar, lemon zest, and lemon juice, adding the juice 1 tablespoon at a time until the glaze is thick but pourable. Add a few more drops of lemon juice to adjust the consistency if needed. While the cake is still slightly warm, pour the glaze over the top, allowing it to drip down the sides. Let it sit for 15 minutes to set.

Once completely cool, transfer the cake to a serving plate and slice. Store leftovers in an airtight container at room temperature for up to 3 days. If you prefer a thicker glaze, let it sit for 5 minutes before drizzling over the cake.