"He's not only singing," Clarkson, 30, told "Entertainment Tonight," "he's marrying us. He totally is. He's done it before with people. He's actually marrying Ashley Monroe from the Pistol Annies, as well."
"You're technically already married when the ceremony happens," first-season "American Idol" winner Clarkson explained. "It's not like it's going to be null and void or anything because it's Blake," she added, laughing.
Clarkson, who did not specify Shelton's credentials or denomination, said she and Blackstock, whom she's been dating since 2011, haven't been able to set a date.
"Our day keeps getting switched because of all the schedules of everybody. We honestly don't have one quite yet. We have the month but we don't have the date."