Preheat the oven to 375°F (190°C). Line a muffin tin with muffin liners or grease it with non-stick spray.
In a large mixing bowl, combine the sourdough starter, granulated sugar, and melted butter. Whisk until smooth.
Add the egg, milk, and vanilla extract to the mixture, whisking until well incorporated.
In another bowl, mix together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, stirring gently with a rubber spatula until just combined. Be careful not to overmix.
Gently fold in the blueberries, ensuring they are evenly distributed throughout the batter.
Divide the batter evenly among the muffin tin, filling each cup about ⅔ full.
Bake in the preheated oven for 18-20 minutes, or until the tops are golden brown and a toothpick inserted into the center comes out clean.
Remove from the oven and allow the muffins to cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely.