Some people naturally pick up the speech inflections of the people who surround them - sometimes within hours of being in company. I deal with Americans a lot as a writer, and I unconciously use American idoms in my speech, simply because it makes communication easier - and I know if I lived there, I'd have an accent in about an hour!
Ms. Stone may have quite a pronounced Devon accent, which would make her incomprehesible in the U.S., so she has probably adapted her speech unconciously.
I personally have never understood her appeal - so she has a 'black' voice as a young teenager ... and ...?