Barker joined regular hosts Konnie Huq, Simon Thomas and Matt Baker on the show for the first time on Friday.
She follows in a long line of names who have hosted the show since 1958, including Valerie Singleton, John Noakes, Peter Purves, Anthea Turner and Tim Vincent.
"I just can't wait to get started," she said. "It's so exciting not knowing what is going to happen next. One day I might be baking bread, the next I could be jumping out of an aeroplane!"
Her family run a bakery in Cambridge, but the philosophy graduate opted for a career in television instead, working as a researcher for BBC Choice and as a runner for shows including Time Watch and Paddington Green.
She made her TV debut as a sports presenter on BBC Choice's Backstage programme.
She does not know how her showreel was submitted to the Blue Peter office.
"The first I knew of the job was when I had a call from the programme inviting me to audition, so someone must have sent the tape in for me," she said.
"It's one of those stories that you think will never happen to you, I still can't believe it."
She watched the programme as a child, adding: "I used to love making everything, and for some reason I always remember Simon Groom and his lumberjack shirts!"
The programme's editor, Steve Hocking, said: "I think Liz is a great find, and I'm sure her warmth and charm will make her a friend to a new generation of the series' viewers."
Katy Hill, who currently presents Sunday sports show Football Fever, left Blue Peter to work on other projects for the BBC.