opened season 21 in epic fashion on Monday, with a show-stopping musical mashup!Before any chairs were turned for the first Blind Auditions of the season, Ariana Grande, Kelly Clarkson, John Legend and Blake Shelton took the stage to remind fans that they've got the talent necessary to coach the next champion.John and Blake kicked things off, showcasing their vocal chops on Sam & Dave's soul classic, «Hold On, I'm Coming,» before Ariana and Kelly brought the house down by blending in Aretha