I'm wondering whether governments will be able to compel ISPs to block access to blockchains or blockchain-based applications. Any thoughts?
The whole idea of a blockchain is decentralization. It's located in computers around the globe. Any computer can join the network and leave it at will. The more there are computers (running full nodes) the safer the network is.
When you start your node you need to connect to some peers which are again connected to other peers etc to form the network. I'm unsure how it's decided which peers you connect to (and how you find the peers), but this is in theory one possible place that can be blocked. But, even if access to those peers is blocked you can just connect to different ones.
So in that sense, no, goverments can't block access to the blockchain. But they CAN block access to services that utilize the blockchain - for example various web3js websites.
You'll always need some link from the 'outside' world to the 'inside' (blockchain) world. Those links can be blocked. Unless at some point we start to live really 'inside' (so out old 'outside' becomes something inside the blockchain).
Anyway, even if access to various links get blocked, it's easy to just open other links. The blockchain is always there, it's just a matter of how to access it.