Blackburn agreed a fee with Chelsea last Thursday after turning down several previous offers, and Duff asked for a few days to think through the deal. The 24-year-old had a clause in his contract allowing him to leave if a club came in with an offer of €24m or more.
"This was probably the biggest decision of my life and I didn't want to rush it," said Duff, who has penned a five-year contract and is Chelsea's record buy. "It was just a matter of thinking everything through. I had seven great years at Blackburn and there was no big reason to move but in the end I decided a big new challenge was necessary."
Earlier on Monday, England defender Wayne Bridge sealed his €9.8m transfer from Southampton FC, with Chelsea's Graeme Le Saux travelling in the opposite direction. Bridge and Duff join other new arrivals Geremi and Glen Johnson at Stamford Bridge in time for the third qualifying round of the UEFA Champions League later this summer.
In total Chelsea's spending has now passed the €52m mark since Russian billionaire Roman Abramovich took control of the west London club.
For the full story on Wayne Bridge's transfer click here
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