The musicians will be among the judges replacing Irish singer Ronan Keating, dance music star Redfoo and Natalie Bassingthwaighte, who have all stepped down following the 2014 series.
Isaak says, “I'm excited to join The X Factor, it is going to be surreal to be on the show as a judge after having watched so many episodes as a fan."
He jokes, "I know it's a show with a history of articulate intelligent judges, and I know they wanted to change that up so they called me. I have watched the show at home and rooted for my favourites, now I will be doing pretty much the same thing... only I will comb my hair and put on a clean shirt. I'm ready!”
And Blunt tweets: "Good news for the UK: I’m being deported to Australia. Bad news for Australia: I’m your new X Factor judge."
They will be joined by Australian singer Guy Sebastian, who returns as a judge after being absent for two series, and the only remaining judge from last year's show, Dannii Minogue.