Although smart phones such as a Blackberry are small computers and can be used to send and receive emails, I highly, highly doubt one would be used as a storage device to back up emails. Smart phones don’t have that kind of storage capacity. Most email applications have a means to back up to large capacity storage such as a hard drive in a laptop or desktop computer, or a server, or what is known these days as the cloud (Internet storage). I think Comey is either incredibly stupid or incredibly duplicitous, maybe a bit of both.
Not familiar with Blackberry, but most email accounts are stored in the the hosting server and updated thru a mail app on the device via a connection method like IMAP. There is no need to backup email on a personal laptop and no need to download the entire account to a smartphone.
Of course with all the nefarious goings on with Clinton and her personal email server, it would figure if the system was backing up to the Weiner's laptop.