First it's important to be clear that blockchains don't necessarily have "an admin". They are peer-to-peer databases and control can be shared between many participants, or even all the participants.
Second any data that is actually embedded on the chain will be visible to all nodes, because they have a copy of it. So if you want to make data visible only to certain parties, you need to encrypt it at the application level, before embedding it inside a blockchain transaction (e.g. using publish in MultiChain). If you do that, it is up to whoever encrypted it to decide who can see the data, by sharing the decryption key as appropriate.
