Stir the flour, sugar, baking powder, and salt together in a large bowl. Beat the egg, milk, and oil together in a separate bowl. Make a well in the center of the flour mixture; pour the egg mixture into the well, and stir until the flour mixture is just moistened. Be sure to not over-mix. Fold the blueberries and walnuts into the batter. Spoon the batter into the prepared muffin cups to about 3/4 full.