The blockchain is in particular trouble because it's decentralized. Every wallet holder would have to re-sign their transactions with quantum resistant key pairs. But this is unlikely to happen to a good number of wallets (eg Satoshi's wallet), meaning a bad actor will crack the wallets that haven't done this one by one. They will then accumulate a truly ridiculous sum of Bitcoin and devalue it for everyone else.

In fact, if someone did develop a quantum computer, they'd probably not announce it - letting it passively crack wallets, slowly accumulating vast sums of wealth, and also decrypting communications at the behest of intelligence agencies. It will likely be years after the development of such a computer that we will actually know it existed.