01 - Preheat oven to 350°F (175°C). Grease a 9x5-inch loaf pan and line with parchment paper for easy removal.
02 - Combine flour, sugar, and cinnamon in a small bowl. Cut in cold butter using a fork or fingers until mixture resembles coarse crumbs. Refrigerate until needed.
03 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
04 - Beat softened butter and sugar in a large bowl until light and fluffy. Add eggs one at a time, beating well after each addition.
05 - Mix in sour cream, lemon juice, lemon zest, and vanilla extract until evenly incorporated.
06 - Gradually add dry ingredients to wet mixture, stirring just until combined to avoid overmixing.
07 - Toss blueberries with 1 tablespoon flour to prevent sinking, then gently fold into the batter.
08 - Pour batter into prepared pan and smooth the surface. Evenly sprinkle streusel topping over batter.
09 - Bake for 50 to 60 minutes until a toothpick inserted in the center comes out clean or with a few moist crumbs. Tent with foil if top browns too quickly.
10 - Cool bread in pan for 15 minutes, then transfer to wire rack to cool completely before slicing.