Start with the crumble topping. Combine melted butter, sugar, flour, and salt in a small bowl. Chill in the refrigerator until ready to use.
Preheat your oven to 350°F (175°C) and line a 1-lb loaf pan with parchment paper, greasing the sides well.
In a stand mixer, mix sugar and lemon zest, rubbing them together with your fingers to release aromatic oils.
Add in flour, baking powder, and salt. With the mixer on low speed, add butter, one tablespoon at a time, until crumbly.
Scrape down the sides of the bowl, then mix in vanilla extract and eggs one at a time. Blend in the lemon juice and milk until smooth.
Toss blueberries in flour to coat, then gently fold them into the batter until evenly distributed.
Transfer batter to the prepared loaf pan and sprinkle extra blueberries on top before adding the chilled crumble.
Bake for about 50-60 minutes, or until a toothpick inserted into the center comes out with moist crumbs. Let cool for 45 minutes before removing from pan.
For the icing, whisk powdered sugar, milk, and lemon juice to a drizzleable consistency. Pour over cooled loaf.
Slice and enjoy!