Absolutely, Backblaze are at fault here. Having a dashboard lie about files being there is a really, really serious bug.

But computers always lie. Your disk says the file is there, you double click it, and it starts making noises and the file doesn't open. Backup systems are the same. It says the files are safe, but unless you test it, they may not be.

In the end, all I want to say is, I wish ZFS was free software :(