Prepare a muffin tin by brushing it generously with soft butter and preheat the oven to 375 F. In a large mixing bowl, stir together the corn meal, rice flour, corn starch, sugar, baking powder, baking soda and salt and whisk to combine. In a separate bowl, whisk together the milk, sour cream and eggs. Make a well in the center of the dry mix in the large bowl, add the egg mixture and pour in the melted butter, whisking just until blended. Let the batter sit for about 10 minutes for the cornmeal, flour and corn starch to absorb the moisture. Stir in the blueberries. Divide the batter among the prepared muffin cups. Bake for about 24 minutes, or until the muffins spring back when pressed gently with your finger.Cool for about 15 minutes – if sticking occurs, gently run a small offset spatula along the inside edge of the muffin tin to loosen, and gently invert muffins onto a wire rack. Serve warm.