Preheat oven to 350°F. Prepare a 9x4-inch loaf pan with non-stick spray.

Zest two large lemons. Place the zest in a large bowl with the sugar. Rub sugar and zest together with fingers to release the lemon flavor. Set aside while you squeeze the lemon juice, then use it for the batter and the icing.

Combine dry ingredients in a small bowl: flour, baking powder, and salt.

Add butter to a large mixing bowl or a stand mixer. Using a paddle attachment or a hand mixer, beat the butter at high speed for about 3 minutes. Add the sugar-and-lemon-zest mixture. Beat at high speed for 3 to 5 minutes until light and fluffy. Add eggs and beat to combine.

While mixing at low speed, add the flour mixture in thirds and the sour cream mixture in halves, starting with the flour and alternating each time. Mix just until almost combined, then add the next mixture, continuing until all ingredients are combined.

Add 1 Tablespoon of flour to the fresh blueberries and toss to coat the blueberries. Fold the blueberries into the thick batter, just until incorporated. Scoop the batter into the prepared loaf pan. Sprinkle with turbinado sugar, if using.

Bake the loaf at 350°F for 63 to 65 minutes, until the top bounces back when lightly touched and a toothpick comes out with a few moist crumbs. Allow the loaf to cool completely before removing it from the pan, or for at least an hour.

While the loaf is cooling, mix the icing ingredients by hand until smooth. Spoon over the loaf once it is cool, or just before serving.