01 - Preheat oven to 350°F. Grease and line a 9x5-inch loaf pan with parchment paper.
02 - In a small bowl, mix flour, brown sugar, cinnamon, and salt. Add melted butter and stir with a fork until crumbly. Set aside.
03 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
04 - In a large bowl, cream the softened butter and sugar until light and fluffy, about 3 minutes.
05 - Beat in eggs one at a time, mixing well after each. Stir in lemon zest, lemon juice, sour cream or Greek yogurt, and vanilla extract until smooth.
06 - Gradually add the dry ingredients to the wet mixture, mixing until just combined. Do not overmix.
07 - Toss blueberries with 1 tablespoon flour, then gently fold them into the batter.
08 - Pour the batter into the prepared pan and smooth the top. Evenly sprinkle the streusel topping over the batter.
09 - Bake for 50 to 60 minutes, or until a toothpick inserted into the center comes out clean.
10 - Let cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.