It could make the root server hosted in one country do so, at which point hopefully the other roots would simply de-list that faulty root server (and start seeking to establish another root server elsewhere).

The root servers (ICANN) are still under the remit of the US department of commerce, so it’s theoretically possible for US legislation at least.

I might be misunderstanding something since I've never been a DNS expert, but wouldn't root servers only be able to block at the level of entire TLDs? Not to say that governments couldn't be interested in doing that, but the flexibility seems limited (without e.g. requiring the root server to return government-approved servers and implementing the blocking on those servers).

Unless they wanted to delist an entire country for some reason it's the TLD servers they would need to go after, not the root servers. And yes, they could attempt to go after the TLD itself and not just the ISP's caching recursive resolver. This is why distributed, censorship-resistant domain resolution is so important. The Internet may be distributed and capable of routing around censorship, but the current DNS system is relatively centralized and thus vulnerable to attack.