Preheat the oven to 350°F / 175°C. Butter and line an 8x4" loaf pan with parchment paper.
In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
Add the sugar to a large mixing bowl, then zest three lemons into the sugar. Use your hands to rub the zest into the sugar.
Add in the butter and use a hand mixer or a stand mixer fitted with the paddle attachment to beat the mixture for 3-4 minutes, until it's light and fluffy.
Scrape down the sides of the bowl. Then, with the mixer running, beat in the eggs one at a time, beating for 30 seconds before adding in the next egg.
Add in half of the flour mixture and gently fold it into the wet ingredients just until the last streak of flour disappears.
Then, add in the milk, sour cream, and vanilla, and gently fold them into the batter until just combined. Add in the remaining flour mixture and fold just until the last streak of flour disappears.
Add the blueberries and 1 tbsp. of flour to a small mixing bowl. Toss the blueberries and flour together until all of the blueberries are evenly coated.
Spread half of the batter into your loaf pan, then top it with all of the coated blueberries. Carefully spread the rest of the batter over the blueberries, then top the loaf off with the streusel.
Bake for 45-55 minutes, or until a toothpick inserted in the center of the loaf comes out with only a few moist crumbs on it.
Let the loaf come to room temperature before slicing, then slice, serve, and enjoy!