Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease it well.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt until well combined.
In another bowl, combine the melted butter, milk, vanilla extract, and egg. Whisk until smooth.
Pour the wet ingredients into the dry ingredients and stir gently until just combined. Be careful not to overmix; it's okay if there are a few lumps.
Fold in the blueberries, ensuring they are evenly distributed throughout the batter.
In a small bowl, mix the softened cream cheese and powdered sugar together until smooth and creamy.
Spoon half of the muffin batter into the prepared muffin cups, filling each about halfway.
Add a dollop of the cream cheese mixture on top of the batter in each cup, then cover with the remaining muffin batter, filling the cups about 3/4 full.
Bake for 18-22 minutes, or until the tops are golden and a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.