In the worst case scenario, the miners that are censoring the transactions own a significant amount of the total hash rate, ignore any transaction including blacklisted addresses and don't add any block to the chain including blacklisted addresses.
A worse scenario would be if governments force the majority of miners to follow their wishes. Any independent miner could still include any transaction, but the blocks they'll find will be orphaned. That's basically a 51% attack of miners, forcing all other miners to follow.
I don't think it's going to happen though. It's much more likely to just ban individuals from trading with certain countries. Makes me wonder if that's going to include signature campaigns on Bitcointalk.