A private blockchain is a blockchain that is not open to the public. A private blockchain is usually permission, meaning that only chosen individuals are allowed to access it.
A blockchain network that operates in a private context is usually controlled by a single entity. This entity can control who has access to the network, as well as what transactions are allowed to be added to the blockchain. In some cases, the entity may also be able to view all transactions that have taken place on the network.
The quality of the code and the ability to run the code on a distributed network depends on the quality of the team of programmers.
The security of a private blockchain is higher than that of a public blockchain because the code is not open to everybody and it is not possible to join the network without the permission of the network administrator.
A private blockchain is more centralized than a public blockchain and this means that it is more efficient but it is less secure.
They are also known as permission blockchains and commercial blockchains.