There just isn’t any.
If there is, I’d really like someone to point it out to me.
If you’re providing a review of something, it might be a one-off, in which case this kind of thing isn’t going to be very important, as you’d hope people would read (or skip) the whole thing, but if you’re regularly producing reviews (as on Val’s blog, which I came across earlier), then you need that review data (in his case marks out of ten) to be part of the intrinsic metadata of your post. At this point I’d normally launch into an RDF speech, but there’s not really any point. RDF would be the natural, and best choice for this kind of thing, but to be honest, anything will do. Anything standard, that all the tools could support. Is the notion of a “Review” just too hard to support? It doesn’t need to specify what kind of review, that should be picked up by other forms of metadata. Just please, someone in one of the big publishing tools, start building in support for writing reviews and adding a score to it so we can start aggregating.