Preheat the oven to 325 degrees F. Line a standard 12-cup muffin tin with paper liners.
In a medium bowl, whisk together the flour, baking powder, baking soda, salt, xanthan gum, and cinnamon. Add the oil, agave nectar, rice milk, and vanilla to the dry ingredients and stir until the batter is smooth. Using a plastic spatula, gently fold in the blueberries just until they are evenly distributed throughout the batter.
Pour 1/3 cup of the batter into each prepared cup, almost filling the cup. Bake the muffins on the center rack for 22 minutes, rotating the pan 180 degrees after 15 minutes. The muffin will bounce slightly when pressed and a toothpick inserted in the center will come out clean.
Let the muffins stand for 15 minutes, then transfer to a wire rack and cool completely. Store the muffins in an airtight container at room temperature for up to 3 days.