The couple, who made their romance official recently, were seen flirting around both on and off the camera, reported People magazine.
"Right when they called for the break, Blake ran over to Gwen's chair. They were talking close and laughing until makeup duties called," a source said.
Before the show began, Stefani, 46, went and hugged the 39-year-old country crooner after being welcomed on set.
Co-judges Adam Levine and Pharrell Williams were also at loss of words looking at the flirtation between the duo.
Levine asked if he knew the song? To which he said, "Gwen sang it to me on the Tonight Show one night, so...