If you went and deleted FB messages individually from within the Facebook app for BlackBerry, then in all likelihood there is no way to recover these messages (that I know of currently). Unless you still have them backed up in your actual email account (Check your email account online to see if the messages still exist). Depending on how you setup email on the BB it's possible that deleting them from the BlackBerry may not completely remove the email messages from your actual email account.
Whatever you do inside the Blackberry (or iPhone) Facebook app, is directly tied to your facebook account, so if you deleted the messages in the app, then those messages will be deleted in your account.
If you just deleted the Facebook app from your BlackBerry, and it then deleted all your messages, then something is totally wrong with the BlackBerry facebook app, as that should definitely not happen.