A block contains one or more transactions. The contents of the block are not encrypted in the blockchain. A block, in general, contains valid and invalid transactions. However invalid transactions have no effect on the State.
A block usually contains three sections: a blockheader, the payload (with at least the transactions) and the metadata section (containing the valid/invalid indicator per transaction).
Fabric: A block in Fabric contains both valid and invalid transactions.
Sawtooth: A block in Sawtooth contains only valid transactions.
Burrow: A block in Burrow contains only valid transactions.
Iroha: A block in Iroha contains only valid transactions.
Block - A set of transactions that are bundled together and added to the chain at the same time.