Kelly Clarkson might wanna give Blake an extra special Christmas gift this year....turns out Blake gave her fiance the nudge to propose.

Blake said, "I told Brandon [Blackstock], 'Man, you need to grow up and figure out that you need to marry this girl,' You need to get your head out of your ass and ask that girl to marry you."

Blake knows them both well...Brandon is his manager.

He's even prepared to work their special day as the wedding singer, "I am going on the record saying I will do the music at the wedding or whatever [they] want to do. I'll be that guy. I will play for four hours if I have to."