It also helps to realize what a Blockade actually is.
I'd suggest looking it up on Wikipedia
True, China view Taiwan as its sovereign territory and does not accept that any other nation has the right to interfere with its attempts to regulate its trade or customs arrangements. Thus the term 'quarantine' which imply that no state of war exists between belligerent nations would be more appropriate (coined by USA with Cuban missile crisis)
In their view this is no different than USA getting in the midst of Spain spat with Catalan. And many would agree particularly since China is the top trading partner of everyone around and more than 120 countries worldwide, which is why I think that unlike the invasion scenario this will get much less international support for USA efforts.
jipehog, physically enforcing the blockade means firing on ships that try to go through. Or boarding them. It will be China who will need to make a decision to escalate and open fire on American vessels, which I doubt because if they are willing to do that, than they may as well start an invasion.
I doubt that China would have any problem in controlling maritime access of cargo ships to Taiwan. While USA could actively try to challenge the quarantine using its shipping but what happens when China simply park several ships in their path what will USA escorts do then?
Better question is would USA dare escalation over Taiwan in the first place if China declares that as redline and such action would lead to escalate into space and nuclear domain?