The 39-year-old "Sangria" singer also revealed that he couldn't wait for fans to see what the 46-year-old former No Doubt frontwoman would be wearing for her performance during Monday night's broadcast.
"Wait til you see! I saw what she's going to be wearing for this performance tonight, it's stunning," Shelton said. "It's so beautiful."
Pharrell Williams, Shelton's fellow coach on the NBC reality competition show, also spoke with ET, where he gushed over Shelton and Stefani.
"I am grateful that Gwen is in the most free place that I feel like she's ever been since I've known her as a human being," Williams, 42, shared. "Not just creatively, but just as a human being, she's floating right now."
"I'm grateful that Blake is smiling these days and inspired as well musically," Williams added.
Shelton also managed to slide in a subtle jab at his frenemy Adam Levine, who has appeared alongside Shelton since The Voice debuted in 2011.
"I'm thankful that Adam grew his hair back," Shelton joked, referring to Levine's controversial bald look he rocked earlier this season. "He was starting to look like Squidward but now he looks more like Eddie Munster again."
Shelton and Stefani reunited on Monday after spending the Thanksgiving holiday apart, when they carpooled to work. Check out the video below for more on the dynamic duo's adorable trip to the studio.