Cream butter and shortening in a large mixing bowl; gradually add sugar, beating until light and fluffy. Add egg, and beat well.
Sift together flour, baking powder, and salt. Add flour mixture to creamed mixture alternately with 3/4 cup milk, beginning and ending with the flour mixture.
Spoon batter into a lightly greased and floured muffin pan, filling three-fourths full. Sprinkle blueberries evenly over batter in each muffin cup. Bake at 375° for 25 to 30 minutes or until lightly browned. Remove muffins from pan immediately. Serve warm.