Preheat an oven to 425 degrees F (220 degrees C). Grease 12 muffin cups.
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.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 15 to 20 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.