Preheat your oven to 375°F (190°C) and line a muffin tin with paper liners or grease with butter.
In a medium mixing bowl, combine the flour, baking powder, and a pinch of salt, whisking them together thoroughly.
In a larger bowl, vigorously whisk the egg and sugar together until the mixture becomes light in color and fluffy in texture. Mix in the melted butter, sour cream, and vanilla extract until well combined.
Slowly incorporate the mixed dry ingredients into the egg mixture, folding gently until the batter is just combined without overmixing. Avoid overmixing.
Gently fold in the blueberries.
Evenly distribute the muffin batter across the cups of the muffin tin, filling each cup to about three-quarters full.
Sprinkle the tops with sugar crystals if using.
Place the muffin tin in the oven and bake at the recommended temperature for 25 to 30 minutes, or until an inserted toothpick comes out clean from the center of a muffin.
After baking, let the muffins cool within the pan for about five minutes. Afterwards, move them to a wire rack to cool completely.