Preheat the oven to 425 degrees F (220 degrees C). Grease 12 muffin cups.
Stir together flour, sugar, baking powder, and salt in a large bowl; make a well in the center. Beat egg, milk, and oil in a separate bowl; pour into the well and stir until flour mixture is just moistened. Do not overmix. Fold in blueberries and walnuts. Spoon batter into the prepared muffin cups to about 3/4 full.
Bake in the preheated oven until a toothpick inserted into the center of a muffin comes out clean, 15 to 20 minutes. Cool in the pans for 10 minutes before removing to cool completely on a wire rack.