I thought you said Dropbox overwrote a file, meaning that file was also being synced by Dropbox, and that Dropbox sync was working. So likely the older file version would also have been synced by Dropbox, which it then overwrote. Dropbox itself keeps old versions of files for 30 days. I think you're saying in this situation Dropbox wasn't syncing though?

My comment was pretty orthogonal to all the Backblaze stuff, which I realize now was confusing.