I don't know a whole lot about backblaze; as they are a backup service, is there a good reason why they couldn't ramp up usage of tape drives? Perhaps keeping recently uploaded backups on-disk, and shuttling older ones to tape?
Their backups are progressive, so a single disk image might be spread across 100 tapes. A restore would be a nightmare. Tapes really only work if you are doing a full image each time.
or a program of partial backups interleaved with full backups, say 3-to-1. You balance a reasonable restore time with a reasonable storage requirement.
Yes. Tapes and tape drives are expensive and awkward to use.
You also have to store them, test them and then find them when a customer wants their data. With drives in a pod they're always online and in a RAID so when a drive dies they can be alerted quickly and customers can access data essentially 24/7.