Preheat your oven to 375 degrees, and line a muffin tin with paper liners. Set aside.
In a large bowl, stir together the flour, brown sugar, cocoa powder, baking powder, baking soda, and salt. Set aside.
In a medium bowl, whisk together the melted butter, buttermilk, eggs, and vanilla. Add the wet ingredients into the dry, and stir gently with a spatula or large spoon. Just before the flour is completely mixed, add in the blueberries and gentle finish mixing. The batter will be lumpy.
In a medium bowl, stir together the flour, cocoa powder, sugar, salt, and melted butter with a fork until crumbs form.
Scoop the muffin batter evenly into your muffins tins. They should be about 3/4 full. Top with a generous sprinkling of the crumb mixture. Bake for 20-22 minutes, or until a cake tester comes out clean. Cool and enjoy!