When I was using BBM 6.0 on my old Blackberry Curve 3, I used to back up my BBM contacts to my email address and SD card using the BackUp/Restore features. When the phone developed some fault which forced me to wipe and reload Blackberry OS on it, I easily restored my BBM contacts from my email address registered with the Blackberry Messenger.
After switching to a new BB powered by Blackberry OS 7, I noticed that there is no more backUp/restore feature in the menu options of the BBM 7 with BBM voice chat. I also noticed that the Blackberry Protect app backs Up phonebook contacts and not BBM contacts. So, I decided to google the new way of backing up and restoring BBM contacts on latest BBs.
Fortunately, I stumbled on an article on Blackberry Knowledge base that explains how the contacts BackUp feature works on BBM 7. Below, is an excerpt from the article:
As of BBM 7 the BlackBerry Messenger contacts are associated with the BBID (BlackBerry Identity). Backing up through BlackBerry Desktop Software will no longer backup BBM as the contacts are not stored locally on the BlackBerry smartphone. When moving to a new BlackBerry smartphone BBM contacts will populate on the device if associated with the same BBID.
In a nutshell, your BBM contacts are now automatically backed up as soon as you login to a BB with your BlackBerry ID. The BlackBerry ID will be used for both backing up and restoring! So, if you buy a new BB and log in with your Blackberry ID, your BBM contacs will be restored onto the new BB.
I hope this helps.
NOTE: Same applies with Blackberry 10 BBM