Gwen Stefani surprised some delighted Blake Shelton fans in Los Angeles on Friday night (Feb. 17) when she turned up onstage at the Forum, joining her boyfriend for a rendition of their duet, "Go Ahead and Break My Heart."

The 47-year-old pop superstar wore a black turtleneck paired with a brightly patterned mini-skirt and thigh-high black boots for the performance, with her hair pulled back into a top knot. Shelton, as usual, kept it simple with blue jeans and a gray button-down shirt, strumming an acoustic guitar as the pair performed the song, which they collaborated on for his If I'm Honest album.

They co-wrote the song early in their relationship, before they'd confirmed it publicly, and were initially a bit reluctant about collaborating.

“We were at a point where it’s like, ‘Do we want to go there? What will people think?'” Shelton tells People. “And then we finally just got to the point where it’s like, ‘You know what? I don’t give a s–t.'”

See Blake and Gwen at the People's Choice Awards

It wasn’t just other people’s perceptions that they were worried about, he says.

“I’m insecure about my writing and who I write with and I don’t know — I’m just weird about it sometimes,” Shelton admits. “I think Gwen has a smaller circle too when it comes to writing, and the fact that we went there that early as we were getting to know each other, it’s just remarkable.”

Stefani also jumped onstage with Shelton to perform the song to celebrate his 40th birthday at Country Jam in 2016.

Stefani joined Shelton for a backstage party after her L.A. surprise, sharing pictures to her Snapchat of a cake with the slogan, "A Cake for Blake" emblazoned on it in frosting, a group of red Solo cups arranged into the letter B and stamps featuring the couple's faces together, according to Entertainment Tonight.

Shelton's Doing It to Country Songs Tour launched Feb. 16 in Bakersfield, Calif., and continues though March. His shows feature two former The Voice Team Blake contestants, Sundance Head and RaeLynn, as his support acts.

See Pictures of Blake Shelton and Gwen Stefani Onstage Together

Watch Blake and Gwen Sing Together at Country Jam