You said you accepted an offer from your dream school, but now UCLA offered you admissions. If you are already into your dream school/program, then why does UCLA's offer mean anything? Is it that UCLA is actually your dream program and that this other school became the dream when you didn't hear from UCLA? Why did you accept from this other school if you hadn't yet heard from UCLA? (This is a functional question - knowing your thought process rather then reprimanding you. There must be a reason; is it because you assumed there was a rejection coming from UCLA, or is it because you really fell in love with this program when you visited?)
But generally speaking, no, you won't get blacklisted. It's early days and chances are the other school is part of the CGS agreement that stipulates that applicants may take until April 15 to accept offers of funding, and may retract acceptances before April 15 without a written release if they get a better offer. Even if they're not, I think most programs understand the vagaries of the process. If you are polite and accommodating, they will understand. I do encourage you, however, to take a moment and make sure this is the decision you actually want to make.
