Does online backup work well for OSX? My external drive recently died, and my local backup failed (a string of bad bad luck). Good thing I have offsite backup, done with one of those cloud backup companies (backblaze). 

Right now I'm requesting for a hard drive mailed to me with my files (4TB). I've never done this before, can I expect the files that come back to be the exact same? I have a hard time believing that because the backup system is not HFS I can only assume? 

In particular, I'm concerned about things like my iPhoto library. In OSX I see it as one big file, but I know in reality it is just a "special" folder. Will the files come back as a proper iPhone library?
The data returned from Backblaze ought to be correctly recognised by the Mac. Folders like iPhoto have a special structure that the Mac shows you as a single file, but in reality it is just a folder.
In the same way, Applications are, in fact, folders too - but the Mac hides the structure from you by default.
This is what your iPhoto Library looks like on Backblaze…

When you get your drive, you may have to point iPhoto &amp; similar apps [Aperture, iMovie, Final Cut Pro all have similar structures for their Libraries] to that 'folder' initially, but it should then be treated as your regular Library.
Edit:
I just tested this empirically by downloading my entire iPhoto Library from Backblaze.
When it arrived, it was in its original disk/folder hierarchy; so had that been an emergency situation I could have copied the entire structure right back to where it came from.
Initially, it looked like a regular folder, but [a step you may not need to take if everything is restored to its original locations] launching iPhoto with the  Opt ⌥  key down to choose a new Library &amp; selecting that folder - as soon as it had finished loading to iPhoto, the 'proper' iPhoto Library icon &amp; 'hidden' structure was restored to it.