There is a reason why not, and it’s that mail clients reply based on the header fields, not the envelope recipients. A separate “blind forward” field in a mail

There is a reason why not, and it’s that mail clients reply based on the header fields, not the envelope recipients. A separate “blind forward” field in a mail client could be a nice addition, but it too falls apart because drafts might be started in one client and completed in another. In practice, deviating from existing MTA and MUA behaviour as specified in RFCs - included the handling of the Bcc header - produces innumerable edge cases and interoperability gotchas, which is why we try not to fuck with them much. The worst case scenario being, all too commonly, some well-intended script installed by a frustrated sysadmin that tries to fix an impedance mismatch and ends up inadvertently creating either a software laser or an open relay.