"I've been with the Scarlets for nine years and I've had some great times there but I felt at this time in my career I needed a new challenge," Rees added.
"Obviously I've worked with [Blues director of rugby] Phil Davies in the past and he's been a great coach - when he was at the Scarlets he was probably the making of me in terms of giving me an opportunity.
"Obviously there's a few other signings he's made with Gethin [Jenkins] coming back from Toulon, and hopefully we can strengthen the squad, put in a good season and hopefully win some silverware."
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