Preheat the oven to 375°F (190°C) and line a muffin tin with paper liners or spray with non-stick cooking spray.
In a large bowl, whisk together 1 ½ cups flour, ½ cup granulated sugar, ½ cup brown sugar, baking powder, baking soda, and salt until well combined.
In another bowl, mix the melted butter, egg, buttermilk, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold until just combined, being careful not to overmix.
Gently fold in the blueberries until evenly distributed throughout the batter.
In a small bowl, prepare the streusel topping by combining ⅓ cup flour, rolled oats, brown sugar, and cinnamon together. Add the cold butter cubes and use your fingers to mix until crumbly.
Divide the muffin batter evenly among the prepared muffin cups, filling each about 2/3 full. Sprinkle the streusel topping generously over each muffin.
Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.
Notes
For a tangier flavor, substitute buttermilk with plain yogurt.