For the streusel
Combine flour, sugars, cinnamon and salt in bowl. Drizzle with melted butter and toss with fork until evenly moistened and mixture forms large chunks with some pea-sized pieces throughout.
For the muffins
Adjust oven rack to middle position and heat oven to 375. Grease and flour 12-cup muffin tin. Whisk egg in medium bowl until pale and evenly combined, about 30 seconds. Add vanilla, sugar and zest and whisk vigorously until thick, about 30 seconds. Slowly whisk in melted butter, add buttermilk and whisk until combined.
Reserve 1 Tbsp flour. Whisk remaining flour, baking powder and salt in large bowl. Fold in egg mixture until nearly combined. Toss blueberries with reserved flour and fold into batter until just combined.
Divide batter in muffin tin and top with streusel. Bake until light golden brown and toothpick inserted into center of muffin comes out with few dry crumbs attached, 23 to 27 minutes.
Cool muffins in tin for 20 minutes, then carefully transfer muffins to rack to cool completely. Muffins can be stored in airtight container at room temperature for 3 days.)
Note: To prevent a streaky batter, leave the blueberries in the freezer until the last possible moment.