Brian from Backblaze here, here is an explanation I typed in elsewhere:
I'm the guy who decided to put things in a folder at the root level of drive. Originally this was a system for EXTERNAL drives that get plugged in then unplugged, then come back later. We need to know what state the backup is in related to that drive, and if the customer has several external drives that come and go we need a unique ID for each one. So I created a top level folder on the external drive called ".bzvol" (hidden) and then placed two files inside of it, a README that explained what the folder was and who created it and why you should not delete it, and a 100 byte XML file that had the drive's unique id AND ALSO the identifier of the backup that owns this drive (some customers have two computers both backed up by Backblaze and they carry a hard drive between them).
There are many designs that would work, in retrospect a better design might be to have used the drive's internal serial number (which is globally unique) and maybe a little mapping database either stored on the Backblaze website or somewhere down under /usr/local or /Library that maps the globally unique drive back to the backup it is associated with.
That's the problem with developing software. After you have worked on a problem for 8 solid years, you are finally qualified to BEGIN working on it and should rewrite it from scratch. :-)