1Preheat oven to 175°C (350°F). Line a loaf pan with parchment paper.
2In a small bowl, toss the blueberries with 2 teaspoons of flour until coated. Set aside.
3In a large bowl, mix the sugar and lemon zest together with your fingers until fragrant and slightly clumpy.
4Add the softened butter and cream until light and fluffy, about 3–4 minutes.
5Beat in the eggs one at a time, then mix in the lemon juice and vanilla extract.
6In a separate bowl, whisk together the flour, baking powder, and salt.
7Add the dry ingredients to the wet mixture, alternating with the milk. Mix just until combined.
8Gently fold in the coated blueberries.
9Pour the batter into the prepared pan and smooth the top.
10For the crumble: In a small bowl, mix together the sugar, flour, cold butter, and a pinch of salt. Use your fingers or a fork to blend until the mixture resembles coarse crumbs. Sprinkle evenly over the batter.
11Bake for 45–60 minutes, or until a toothpick inserted in the center comes out clean.
12Cool in the pan for 10 minutes, then transfer to a rack to cool completely before slicing.