In a small mixing bowl, cream butter and sugar. Add egg and sour cream; mix well. Combine the flour, baking powder and salt; add to creamed mixture alternately with milk. Fold in blueberries and rhubarb.
Fill 12 greased or paper-lined muffin cups about two-thirds full. Bake at 400 degrees F for 20-25 minutes or until a toothpick comes out clean. Cool for 5 minutes before removing from pans to wire racks.