China became the world's largest oil importer last week, and sure enough here's an article considering the ins and outs of an oil blockade as a short-of-war form of compulsion against the PRC. I'm sure the PLAN have done the same, from the other side of the hill. A couple of points a) it's a huge job requiring much wider diplomatic co-operation than China's rivals in various South/East China sea disputes. b) the linchpin here is apparently Indonesia, with which as far as I know China doesn't have any territorial issues.
As for counter measures, I recall that someone published a few years back a book which demonstrated quite effectively that it's next to impossible for people on ordinary incomes in the US (or Europe for that matter) to live for a year without making use of goods made in full or part in China. I'm not sure if that's true anymore, but it's kind of awesome just to contemplate the idea that China could stage an effective counter-blockade by simply refusing to export anything at all. Of couirse, that would be a side-effect of an oil blockade as well.