The boyband - individually known as Duncan James, Lee Ryan, Antony Costa and Simon Webbe - joined the cast for a rendition of 'Who Loves You' by The Four Seasons last night (October 11).
Blue also posed for pictures with the stars of Jersey Boys and Children in Need's famed mascot Pudsey.
Cameras also followed them backstage at the Prince Edward Theatre in London, where they were snapped helping each other scrub up for the show in their make-up room.
'Pop Goes the Musical' has already seen Alexandra Burke perform in Les Misérables last month.
Geri Halliwell will join the cast of Singin' in the Rain on October 25, while Macy Grey and Stooshe will appear in Thriller on November 15.
Tickets are available via lastminute.com and are priced at £80. They include a top-price seat, signed programme, Pudsey bear, complimentary drink and VIP area access.
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