As far as I see it at the moment, there are no secure blockchains, only those that are more expensive to hack then others. See “how much does an attack on a blockchain cost right now”.

Quantum Computing might make a true distributed ledger possible in the first place, due to its tamper-detecting nature. On the other hand, I have a hunch that you don’t even need Quantum Computers to ruin the blockchain’s day — a certain type of computers that have been in the labs since the 1970s look like they should already be able to do this — but I haven’t done enough research to be able to confirm this yet, so unlike the rest of what I wrote this is pure speculation.