The Honey Bee hitmaker celebrated turning 40 at the Country Jam festival in Grand Junction, Colorado on 18 June (16), and Gwen helped her man mark the occasion by joining him for a duet of their track Go Ahead and Break My Heart.
She also delighted the crowd with a rendition of her band No Doubt's hit Hella Good, but Blake is convinced she rocked out a little too hard and made his low-key set look awful in comparison.
"I'm doing my normal, you know, what I think is rockin'," he explained to U.S. talk show host Chelsea Handler. "Standing up there with my acoustic guitar in front of my stand and singing my songs.
"I had my band work up one of her songs, Hella Good. And I thought, 'Man, that'd be so cool to have her come out and surprise'. It's the worst thing I could have done because all it did was make me look terrible!
"Literally 40,000 were jumping up and down," he continued. "I mean, that's probably what it's like at real concerts, but I was like, 'Whoa. What's going on?' People were so excited to see her."
Blake only has himself to blame after hyping up Gwen's performance to the crowd.
He told the audience, "This is fun! Y'all have been so freakin' nice to me on my 40th birthday and I guess if I had one birthday wish for me personally... I wish I could rock. I wish I was one of those type of artists that could rock but I can't."
Clearly egging Gwen on, he continued, "For my birthday I wish I could hear an artist who could rock, that's my birthday wish."
The couple began dating last autumn (15) after Blake divorced Miranda Lambert and Gwen's marriage to British rocker Gavin Rossdale came to an end over the summer.