Katy Perry – review

As a piano plays mournfully beside her, Katy Perry is dressed in black and singing a song about suicide. It's not what you expect from a pop star famous for her cartoonish style and bouncy hymns to self-empowerment. But with, as Perry points out, "thousands of people tuning in online" for the last night of the iTunes festival, what better time to reveal a reinvention?

Gone are the candy coloured hair and playfully provocative costumes of 2010's hugely successful Teenage Dream. It's not the au naturel, ethereal look of the soon-to-be-released follow-up Prism that Perry debuts, however, but a very Camden Town combination of safety-pin adorned leather leotard, tartan miniskirt and boots; her bunched wavy hair finishes off the schoolgirl grunge look. Her powerful voice is hard-edged and has never sounded better, avenging on Part of Me, almost vindictive and wrapped in stuttering guitar on I Kissed a Girl.

But the geek-turned-rock chick is just one of the facets of what Perry calls her "Prism preview". Walking On Air is a homage to 90s rave that has Perry and her eight backing dancers striking poses with a white sheet and a wind machine, before she asks permission "to get a bit down and dirty", on the brooding, hip-hop-flavoured Dark Horse.

But it's the startling piano ballad By the Grace of God – played here for the first time – that shows how far Perry's moved from her Technicolor past. Joined centre stage by two backing singers, she details how the collapse of her marriage to Russell Brand left her her contemplating the end. "Looked in the mirror and decided to stay," she sings, her voice growing from heartbroken whisper to soulful rebuke, "wasn't gonna let love take me out that way."

It's a bold and brave move from the queen of perky innuendo, who seems relieved to get back on familiar ground with the confetti-sprinkled Wide Awake and spine-tingling singalong Fireworks. She ends her all-too-brief, 45-minute set with the gloriously bombastic, recent No 1 Roar, Perry's huge smile and the crowd's ecstatic reaction proof that she's done with blue wigs and foam guns for good.