Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease it lightly with cooking spray.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
In another bowl, mix the melted butter, egg, vanilla extract, and buttermilk until smooth.
Pour the wet ingredients into the dry ingredients and gently stir until just combined. Be careful not to overmix; a few lumps are okay.
Fold in the blueberries gently, ensuring they are evenly distributed throughout the batter.
Divide the batter evenly among the muffin cups, filling each about 2/3 full.
Bake for 18-22 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the pan for 5 minutes, then transfer them to a wire rack to cool completely.