I am wondering if the content (transaction) of the consortium blockchain or Quorum blockchain can be visible only for the validators. In other words, the registered users (who can access the nodes) cannot read the content of the blockchain. Is this case possible on the consortium (or Quorum) blockchain?
Accoring to Quorum documentation:
So called 'Private Transactions' are those Transactions whose payload is only visible to the network participants whose public keys are specified in the privateFor parameter of the Transaction . privateFor can take multiple addresses in a comma separated list.
so the transactions are visible to everyone that has an access to a participating node through ipc or rpc API - a public txns payload will always be visible from node A, but for private txns, node A had to be part of the privateFor for the txn in question to be able to access payload from node A.
Regardless if this was a private or public txn, the txn needs to be accepted into a block and the validators a are nodes that agree what goes into a particular block - they don't need to know or see the payload of the txn.