Preheat oven to 400. Spray 12 muffin cups with non-stick spray.
Remove the package of blueberries from the mix and set aside. Place the muffin mix in a large mixing bowl and make a well in the center.
Place the yogurt, eggs and vanilla in a small mixing bowl and stir with a fork to combine and break up the egg yolks. Pour the yogurt mixture into the well in the muffin mix.
Stir the wet and dry ingredients together just until combined, about 20 strokes.
Pour the reserved blueberries into a strainer, rinse them under cold water and drain well. Fold these into the batter along with the fresh berries. Stir another 10 strokes, just to combine. The batter will still be a little lumpy.
Spoon 1/3 cup batter into each muffin cup, filling it three quarters full.
To prepare the streusel topping, place the brown sugar and flour in a small bowl and stir together. Add the butter, mashing it with a fork until the mixture is crumbly. With your hands, sprinkle a heaping teaspoon of this topping on top of each muffin.
Bake the muffins until they are lightly golden and just spring back when lightly pressed with your finger, about 20 to 23 minutes.
Remove from oven and let cool 5 minutes on a wire rack before removing from pans. Cool for an additional 15 minutes before serving.