Blackberry was killed by their failure to upgrade their infrastructure.
Do you guys remember when they lost all emails, not once but TWICE in a matter of a week? That was what got businesses to say "oh shit, this isn't something we can depend on" and get other phones working. I'll bet that they're still running all their services through that same fucked up server in Ontario, despite the failure they've had on the unit.
Once that seed of doubt got planted, compounded by the fact that people could start using their personal phones (i.e. free to corporate) for business, that was it. Stick a fork in them, they're done. The one thing they said they were good for they couldn't do anymore.
Of course, given that they were hilariously spied-on and infiltrated (not as much, but almost as badly as Nortel), who's to say if those failures were accidents or if they were pushed?