Is the model of the phone known? If it's a relatively new BlackBerry running Android, and if it can be rooted:

  * the main WhatsApp msgstore database in /data is not encrypted
  * the msgstore backup databases (.crypt* in /sdcard) can be decrypted easily using the key file (mentioned in the article) which is also stored in /data

One could probably reverse engineer the WhatsApp APK to figure out how the key file is generated.

I would hazard a guess that Signal messages are also not stored encrypted at the source and destination (beyond the protection offered by the operating system).