Backblaze randomly stopped backing up for many Mac users on 3/13 (myself included), and the only information about this problem and its fix is in this reddit thread(!?)
https://www.reddit.com/r/backblaze/comments/1jdlzop/bb_stopped_backing_up_after_macos_1532_update/
tl;dr:
1. Restart the computer
2. Manually update the app via the latest installer
3. Invoke a magic command to make it rescan your drive(!?)