Preheat oven to 425°F.
Prepare a muffin tin with nonstick cooking spray.
Whisk flour, baking soda, and salt together in a large bowl. Set aside.
In a stand mixer, mix together the sourdough starter, milk, egg, butter, and brown sugar.
Remove bowl from mixer and fold in the flour mixture. Once mostly combined, gently add the blueberries into the batter.
Use an ice cream scoop or large spoon to divide batter equally among the prepared muffin cups. Sprinkle tops of muffins with granulated or coarse sugar (optional).
Bake until muffin tops are golden and just firm, about 25 minutes, at 425°F.
Cool muffins in muffin tin for 5 minutes, then transfer to wire rack.