Preheat oven to 400 degrees F. Line a muffin pan with paper muffin cups or spray with nonstick cooking spray. Set aside.
To make the streusel: Combine the flour, brown sugar, and butter pieces in a small bowl. Mix in the butter with your fingers or a pastry cutter until you have a crumb mixture. Place the streusel in the refrigerator until ready to use.
To make the muffins: In a large bowl, whisk together the flour, sugar, baking powder, and salt. Add the brown sugar and whisk again until there are no brown sugar clumps.
In a small bowl, combine the melted butter, eggs, buttermilk, and vanilla. Whisk until eggs are well combined with the other liquid ingredients.
Pour the liquid ingredients over the dry ingredients and gently stir together with a spatula. Don't over mix the batter, stir just until flour disappears. In a small bowl, combine 1 cup of the blueberries with 1 tablespoon of flour and toss until blueberries are coated. Gently fold in 1 cup of the blueberries, being careful not to over mix.
Divide the muffin batter evenly among the muffin cups. Top each muffin with remaining 1/4 cup blueberries, pressing them gently into the tops. Remove the streusel from the refrigerator and sprinkle evenly over each muffin. Gently press the crumbs into the batter.
Bake the muffins for 18 to 20 minutes or until the muffin tops are golden and the tops are set. You can insert a toothpick into the center of a muffin and if it comes out clean, they are done.
Transfer the pan to a cooling rack and let the muffins cool for 5 minutes in the pan. Remove from the pan and enjoy!