Yes, blockchain can be hacked. No matter how secure and how powerful blockchain might be, they can still be hacked because hackers look for a weak link in the security and exploit this link to hack any website, bank or even blockchain. Hackers will use a Sybil attack to send a large number of nodes to a single network, owned by a single party. This will attempt to disrupt network security by flooding the network with millions of bogus transactions to disrupt the network and the security. They might use a Routing attack to disrupt the ISP on the network. There are several possibilities that a hacker can use to hack any secure network and blockchain is no exception.