OK, so in Hestia control pane, go to Server (cog icon) > Configure (button)
https://myhost.com:8083/edit/server/
In the backup section, you can select Remote and then Backblaze, and then you have three boxes to fill in.
Bucket
Application ID (KeyID)
Application Key
You need to set up a bucket first in Backblaze and issue an Application Key for it, obviously.