Combine 1 1/3 cups sorghum flour, granulated sugar, sweet rice flour, potato starch, baking powder, and salt in bowl of large (14 cup) food processor. Pulse to combine. Add the butter. Pulse until no large pieces of butter are visible. The mixture should appear uniform. Add the eggs. Pulse to combine. Mixture will be dry. Will the food processor running, add the milk and vanilla through the feed tube. (If your food processor doesn't have a feed tube, add the milk in four additions, blending thoroughly between each addition. Add the vanilla with the final addition of milk.) Once you've added the milk, mix the batter for one minute. Using a flexible spatula, transfer the batter to a large mixing bowl. Toss the blueberries with remaining sorghum flour. Gently fold in the blueberries.