A single government can easily block it in country — nobody needs or wants Bitcoin, they want to buy things and that can be attacked by targeting whoever participates in the network. Bitcoin is designed to make this extremely easy since that public ledger means they can go after ever transaction you’ve ever made at any point in the future, not just at the time of transaction. Unless the government is incredibly corrupt and powerless nobody is basing a black market on a system which giftwraps your transaction logs for prosecutors.
A government which participates in international banking coalitions or treaty groups can use those relationships to go after transactions internationally, too. Think about how drug cartels have secret bank accounts frozen, and then about how Bitcoin is designed to make that easier by giving the authorities a full list of your activities which is trivially extended to every partner. Once you’re on that list, everyone you know will be getting pressure to turn on you.
Now think about a major government: they can not only use normal law enforcement but also control the exchanges themselves with things like anti-money laundering laws. Someone might choose to blow off demands from a small country or a pariah state like Iran but if the US or EU decided to act everyone involved is now facing things like not being able to fly internationally or do business with most major companies. Since they have no upside to resisting and it’s trivial to block Bitcoin transactions, anyone in those jurisdictions is unlikely to risk that protecting a stranger in a different country.