01 - Preheat oven to 350°F. Grease a 9x5-inch loaf pan and line with parchment paper.
02 - Combine flour, sugar, and cinnamon in a small bowl. Cut in cold butter until coarse crumbs form. Refrigerate until needed.
03 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
04 - Beat softened butter and sugar until light and fluffy. Add eggs one at a time, mixing well after each.
05 - Incorporate sour cream, lemon juice, lemon zest, and vanilla extract into the butter mixture.
06 - Gradually add the dry mix to the wet ingredients, stirring gently until just combined. Avoid overmixing.
07 - Coat blueberries with flour and fold them carefully into the batter.
08 - Pour batter into the prepared pan, smooth the surface, then sprinkle streusel evenly over the top.
09 - Bake for 50 to 60 minutes, or until a toothpick inserted in the center comes out clean or with moist crumbs. Tent with foil if browning too fast.
10 - Let the loaf cool in the pan for 15 minutes, then transfer to a wire rack to cool completely before slicing.