I think blackberry's best bet is to power forward with BBM messaging and be the super messaging platform everyone needs. Charge $5 a year, get 1 billion users, and then branch out to doing other super specific blackberry things on other platforms.
Only focusing on messaging I feel could be a risky thing. Especially if they charged a subscription fee. It would have to be quite an amazing messaging app to convince people to pay $5/year. Even if they do manage to convince 1 billion people to pay $5/year. What happens when a startup does it for free?