Where are iCloud files located in my BackBlaze backup? 

If your local folders (such as Documents) are synced to iCloud Drive, their files will appear in their normal locations on your computer (i.e. /Users/[username]/Documents) and will be backed up there by Backblaze. See this guide for assistance in navigating through your backup.

Any other iCloud Drive files that are not synced from your local folders (like Documents or Desktop) can be found at /Users/[username]/Library/Mobile Documents/ and should exist in this same location in your backup. As a note, /Users/[username]/Library/ is usually a hidden folder on your computer, but should show up normally in your backup.