you can use the blockchash() function to query the hash of a block from the blockchain. However, it is important to note that the hash of the latest or actually block is actually not known in execution time, because the block is still being minded. As a consequence block.blockhash(block.number) results always null.
Otherwise the blockchash might be zero for old blocks as well, as Ethereum stores only the last 256 block hashes for scalability reasons.
Otherwise the blockchash might be zero for old blocks as well, as Ethereum stores only the last 256 block hashes for scalability reasons.
