I think establishing a blockade is generally considered an act of war. Possibly a war crime if the intent is to starve civilians? (Not sure, this is way outside my wheelhouse). I’d expect it to be about as escalatory as using US ships to attack the mainland, more or less.
Is the history of China any more famine stricken than any other region if the world? In recent history (last two centuries) maybe, but before that? Typically you don't become the worlds most populous country by having frequent famines.
