Pre-heat oven to 400 degrees F. Line a muffin pan with liners, or spray with oil.
In a small bowl, mix together 2 cups of flour and baking powder.
In a large bowl, mix the milk, sour cream, sourdough discard, eggs, and vanilla together until well combined. Add in the flour mixture, and mix until batter is smooth.
In another bowl, toss blueberries in 1 tablespoon of flour, making sure to coat well.
Gently fold in the blueberries, using a spatula or wooden spoon.
Mix together the ingredients for the streusel, until you have a crumbly consistency.
Evenly divide and spoon the batter into the muffin liners. Sprinkle the streusel mixture over the tops of the muffins.
Bake for 20-24 minutes, until golden brown.
Remove muffins from oven and cool on a wire rack.