It's definitely a London thing. They became hugely popular in the summer of 2009, rapidly making their way into the constantly shifting memepool that is the grime scene.
Witness "Maxwell D - Blackberry Hype": http://www.youtube.com/watch?v=2qtShEs5LQs
Believe it or not this isn't paid product placement. Hence he lists RIM's competitors in the intro. They probably still sent maxwell some units though.
EDIT: BBM was also heavily used as a way of sending shouts to pirate radio stations. You couldn't really ask for better marketing. It's very much waned now though.