Nothing prevents records on a blockchain from being encrypted. The blockchain simply guarantees that the record is genuine, its contents are not relevant to the blockchain itself.

Nor is it impossible to imagine private ledgers that are closely held by some group of people for their own purposes. These can be subject to any and all of the IT safeguards any other data store can be.

Its best to look at blockchain as simply a technique by which records can be appended to a file in such a way that everyone with a copy of the file agrees that those particular records, in that particular order, form the canonical version of that file, and an agreed-upon process for appending new canonical records exists. Nor should we be too narrow in our conceptions of how such ledgers can be stored. For instance it would be perfectly feasible to store a blockchain as a series of records in an RDBMS, along with the cryptographic fields needed to link the records in the chain (as just an example, I don't know if there's a good use case for this or not).