I wrote a WebDAV server you can use as the frontend for it that can encrypt everything you upload.
https://hub.docker.com/r/sciactive/nephele
There’s something I’m using that Backblaze doesn’t support, so I’m fixing that right now.
Edit: Ok, I fixed the issue and updated it on Docker. It now works with Backblaze. It’s a bit slow, but I’m not sure if that’s just my connection or Backblaze.