> Blu-ray, or any non-rewritable optical media, guarantees that it hasn't been tampered with.

In terms of external tampering, I think that need is better-served by encrypting everything, which you probably want to do anyway. Or at least using some sort of checksum signed by a key.

However if the focus is "what if the backup process is compromised", then yes, having some date which is beyond its writing-reach may be useful. Another aspect of this might be a remote-backup system where one set of credential is enough for daily backups and restore, while another is needed in order to access a set of older duplicates that are remotely maintained as ransomware-protection.