And yes. I'm using the engineering definition. I don't believe in letting a gaggle of marketers and scammers define my terms. A blockchain is a specific technology. It doesn't mean 'whatever scam is happening this week', even if said scam involves a blockchain.
I don't blame you for associating blockchains with scams and fully open projects, that's undeniably what we've seen it used for. But that's not what defines a blockchain.
"A scalpel can only be used for surgery"
"If you use a scalpel to cut a steak, it's still a scalpel."
"There must be some new aspect to scalpels! We've been able to make steak knives for decades!"
> The analogy is [the P2P application] on a LAN.
The analogy is the P2P application where regular clients can only discover a Special Master Client that must be running on a fixed IP, which only permits connections if you have credentials for a user-account arranged in advance.
In each case, the system's centerpiece feature is being voided, but that feature is different between them.
1. For "Blockchain", the centerpiece is unrestricted participation. (Other decentralization is an indirect effect.)
2. For P2P file sharing, the centerpiece is how nobody needs to run an expensive/vulnerable central server, but it wasn't a contradiction in terms to have a private peer-network.
A blockchain is ledger shared between multiple computers. Entries in this ledger contain the cryptographic hash of the previous entry, creating an append-only data structure where existing records cannot be modified.
While blockchains are generally used in p2p and open networks, this isn't a requirement. Bitcoin was a blockchain when Nakamoto was the only node. Popularity and openness are not required to meet this technical definition.
If we charitably assume "blockchain" has some engineering meaning (and it isn't purely a word for marketing/scamming) then there's some new aspect which sets it apart from the distributed-databases we've been able to make just fine for decades.
Uncontrolled participation is that key aspect. Without that linchpin, almost all the other new stuff becomes weirdly moot or actively detrimental.
> If you spin up a private bitcoin network, it's a blockchain even if nobody else knows or cares about it.
That's practically a contradiction in terms. It may describe the ancestry of the project, but it doesn't describe what/how it's being used.
Compare: "If you make a version of Napster/Gnutella with all the networking code disabled, it's still a Peer-to-Peer file sharing client even when only one person uses it."