Since the present time bitcoin made, I dont hear a report the blockchain is hack by unknown. I just read trade or wallets hack. I mean bitcoin can hack the main hackable is the exchanger like poloniex, bittrex or wallet like coinbase, electrum and some more. Which hacking into those sites to take the bitcoin is entirely conceivable. We should secure our particular record from the attacker soon as possible consistently your will change your secret word every month.
Bitcoin might be focusing on their security, no one will tell that it cant be hacked. But there is a small chance that bitcoin be hacked. Users of bitcoin, their coins can be hacked thru their wallet not in blockchain. Wallet apps must improve their security for the users to become safe from havkers.