Preheat oven to 425F. Line two 12-cup muffin tins with 16 muffin liners. Set aside.
In a large bowl, whisk melted butter, sugar, agave, sour cream, eggs, milk, and vanilla until smooth. Add flour, baking powder, and salt to bowl and gently fold until batter is just combined. Gently fold in flour-coated blueberries. Fill prepared muffin tins 3/4 full of batter. Set aside.
Make the streusel: In a small bowl, combine all streusel ingredients until crumbly. Sprinkle over filled muffin tins and gently press to adhere crumbs to batter.
Bake muffins at 425F 5 minutes, then reduce heat to 375F and continue baking an additional 13-15 minutes until muffins test done with a toothpick. Cool in pan 5 minutes then transfer to a wire cooling rack. Enjoy muffins warm!