Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or grease well.
In a large bowl, whisk together the gluten-free flour, baking powder, xanthan gum (if using), and salt.
In another bowl, beat the sugar and eggs until light and fluffy. Add the oil, milk, and vanilla extract, mixing well.
Gradually add the wet ingredients to the dry ingredients, stirring gently until just combined. Be careful not to overmix!
Fold in the blueberries and lemon zest (if using) gently.
Divide the batter evenly among the prepared 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 before transferring to a wire rack to cool completely.