It seems that quantum computers will have very good performance at the calculation of hashes that means that they possibly can outperform existing mining hardware.
Hacking of blockchain depends on its type of consensus. If it's POW (like at Bitcoin) then to hack blockchain it's required to possess at least 51% of overall computational power. So if someone will have very powerful quantum computer that outperforms joint computational power of all miners, then he can add fake data to blockchain (to hack the database)

If consensus mechanism is DPOS (like in Steemit) where blocks are validated by witnesses, then even having big computational resources will not help to hack it.

I'm not an expert, so I hope someone will correct me if I said something wrong