From the beginning, this show has shone more of a spotlight on its judges than its contestants – and while no winner of The Voice has yet become famous, such judges as Blake Shelton, Adam Levine and others have gotten big career bumps, and widened their fan bases, by spinning around in those big red chairs. And tonight, a guest mentor arrives to steal even more of the spotlight: Taylor Swift, whose shift from country to pop with her just-released CD, and her Number One hit “Shake It Off,” ought to make any byplay with country star Shelton quite interesting.