The preparations for this service are said to be in it's final stages and supposed to be launched as early as in the coming weeks. There will be access to up to 50 songs at a time, which can be listened to on your own device or shared with your friends over BBM.
The service is said to cost significantly lesser than it's competitors and will surely view some new users over to the BlackBerry platform.
With it's lower price-tag and limited song selection, the indication is that BBM Music is aimed at a younger crowd, and is not aimed as a competitor to iTunes. It will rely on it's strength of sharing over BBM to solidify its place among its competitors.
source - WSJ