I would like to say that the blackberry is highly obsolete and pretty much dead, nobody longer use such smartphones.
No, this tag should not be deleted. Just because blackberries are obsolete doesn't mean that there can't be questions about it. In general, deleting a tag should only be done if there are good reasons to do so.
Does it describe the contents of the questions to which it is applied? And Is it unambiguous?
blackberry fulfills both of these. It's pretty clear that questions with this tag are about blackberries.
Is the concept described even on-fopic for the site?
Yes, questions about the security of blackberry devices are on-topic, as long as they meet all the other requirements.
Does the tag add any meaningful information to the post?
Does it mean the same thing in all common contexts?
Depends. It may either refer to the device or the operating system, but they are closely related enough to be functionally interchangable, similar to android. It's clear that these questions are about the security of Android devices, not about general Android questions.
Having gotten that out of the way, ask yourself if the existence of the tag causes any problems. I would wager that it doesn't. It's not like security, which adds absolutely nothing at all, or policy, which is so ambiguous that adding the tag doesn't really describe anything.