If you have control over 50% of the nodes then it can be hacked. But Blockchain is not centralized. The ledger is distributed across nodes all over the world. So practically it is impossible to hack blockchain. But I've heard that a quantum computer may hack the blockchain system. I have to study it more in order to understand the quantum computing process.
Blockchain can not be hacked but it can be 51% attack, you can not hack blocks itself but you can control the validation of blocks which means you can upload cheating blocks against the chain and double spend ect.. ..
As for quantum computer they can not do anything to blocks but they are related to wallets and hacking ( private keys ) .