Ya i wouldnt be so sure it cant be blocked.
It can be blocked but only by force and the only countries that can do this are the US itself, the baltics, the scandinavian countries, occupied Korea or Japan, neither of which could attack Russia/China without a high likelyhood of getting nuked in return. Meanwhile, the Suez can be closed and there is nothing Russia or China could do (other than going around or making more trains).
As long as it's not blocking Russian ships from the Black Sea the US can do anything and not much would come out of it straight away. Some countries would try to become independend of the canals but that would likely not lead to war.
On the other hand, the US literally shooting at Russian or Chinese ships would be a declaration of war that would require an immediate response of those attacked else it would just escalate to them losing more ships and probably even their ports and coastal infraestructure/cities being targeted soon as well.