The British royal joined the two musicians on stage at the Winter Whites Gala at Kensington Palace last week, shortly after James' own performance.
The 'You're Beautiful' singer has admitted he would have liked to duet with William.
Speaking at Paul Newman's SeriousFun Children's Network Gala at The Roundhouse in London last night (03.12.13), Blunt told BANG Showbiz: ''I was on the same stage but no I was just before, he [William] didn't jump on stage when I was on there.
''I didn't hear him, sadly I missed it. I would have liked to have heard that.''
Blunt, 39, recently released his new album 'Moon Landing' and hit single 'Bonfire Heart' and he has been pleasantly surprised by the positive response the record has received.
He said: ''It's been amazing, I didn't expect to be in the UK, and I didn't expect to be a single seller at all, so to have a song in the top five or wherever it was, was amazing for me.
''Who knows how these things go but at the moment I'm enjoying myself.''
Other stars at the event included Keira Knightly and her husband, Klaxons singer James Righton, François Cluzet, tennis legend Boris Becker and wife Lilly Kerssenberg.
Blunt performed on the evening and entertainment was also provided by Kate Nash and The Who's Roger Daltrey.
The SeriousFun Children's Network was founded by Newman - who died in September 2008 - in 1988 and is a global community of 30 camps and programmes serving children with serious illnesses and their families - always free of charge.
The gala raised £500,000 which will be used to support SeriousFun's camps and programmes around the world, enabling the charity to serve children at no cost to their families and to expand their reach.
He also slapped a jet-lagged Taylor Hawkins.
The acclaimed performer had just cancelled touring due to a battle with cancer.