With the important exception that there are no blocks. If you're doing transactions one at a time, it's just a long ledger with checksums. Which are just a timesaving techinque so you don't have to replay log segments and compare with snapshots.

In practice, many of these organizations talking about using in-house blockchains have legal trust relationships and systems of laws in common, and they won't only be relying on a transaction log to make sure the record isn't getting fiddled.

And often the threat model isn't related to changing the historical ledger anyhow; there is a lot of fraud, embezzlement, and other financial crime that attacks other points in the system. That requires a significant system of financial controls, which will be sufficient to catch ledger tampering as well. Meaning that the checksums become a modest nice-to-have, not a significant anti-fraud technique.