RIPA again
Whether the Police looking at a Blackberry archive is interception or not is actually very much more complicated than what has been said here. I won't go into the gruesome details, but it could go either way.
However, even if it is interception, the Police can still do it. According to RIPA S.1(5)(c) interception is lawful if "it is in exercise, in relation to any stored communication, of any statutory power that is exercised (apart from this section) for the purpose of obtaining information or of taking possession of any document or other property".
The police would be looking at stored communications if they looked at a Blackberry archive, presumably using a statutory power under PACE. The question of whether it's stored so the intended recipient can access it or not doesn't come into it at all.
That may be an important distinction for journalists and ordinary people, but not for the Police - if it's a stored communication they can access it under PACE, no matter whether doing so is interception or not.
There is another question to be looked at though - Would Blackberry keeping an archive of messages be interception? Undoubtedly, yes it would be, see S.2(2)(b).
Would that interception be lawful? S.3(3)(b) says it would be if "it takes place for purposes connected [...] with the enforcement, in relation to that service, of any enactment relating to the use of postal services or telecommunications services."
Afaik there is no enactment forcing Blackberry to keep Blackberry messages, so it's probably illegal for Blackberry to keep an archive [*]. But not the Police to look at it.
[*] unless it's "for purposes connected with the provision or operation of the service" - eg an archive which intended recipients can access. However if Blackberry started keeping an archive specifically so it could be accessed by the Police then they would be intercepting, and doing so illegally.