Preheat the oven to 375℉. Line a 12-cup muffin pan with paper liners or lightly grease it.
Make the crumble: Combine flour, sugar, cinnamon and salt. Pour in melted butter and stir with a fork until coarse crumbs form. Set aside.
Combine dry ingredients: In a large bowl, whisk together flour, sugar, baking power and salt.
Combine wet ingredients: In another bowl, whisk the eggs, melted butter, milk and the vanilla until smooth.
Make the batter: Pour the wet ingredients into the dry ingredients. Stir gently just until combined. Don't overmix- the batter should still be slightly lumpy.
Add blueberries: Toss the blueberries with 1 tablespoon flour, then gently fold them into the batter.
Fill muffin cups: Divide the batter evenly among the 12 muffin cups, filling each about 3/4 full.
Add crumble: Generously sprinkle the crumble topping over each muffin.
Bake for 20-25 minutes, or until the tops are golden and a toothpick inserted into the center comes out clean or with few moist crumbs.
Cool: Let muffins cool in the pan for about 5 minutes, then transfer them to a wire rack.