What does restore mean this context? I’ve downloaded files from their online portal without providing a key. Perhaps restoring in this context means having them mail you a hard drive.

Yeah I mean you gotta trust them on some level. Backblaze could also push a client update that nerfs the encryption. If you've got really sensitive data I'd probably pick something else.

You can always encrypt yourself and rclone to b2. Likely cheaper if you are not a data hoarder.