In a small bowl, cream butter and sugar until light and fluffy. Beat in egg and sour cream. Combine the flour, baking powder and salt; gradually 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° for 20-25 minutes or until a toothpick comes out clean. Cool for 5 minutes before removing from pans to wire racks. Serve warm.