The page notes: some people press return once between paragraphs, and some press return twice. The same happens between one post and the next. As far as I know, this is a matter of personal taste, and there isn't a rule about it.
There is a guideline on EnWiki and probably elsewhere: LISTGAP. It is desirable to avoid blank lines in the middle of colon/star/hash lists because text-to-speech screenreaders do not handle the gaps as smoothly. (They speak something like "end list new list" instead of "next item", and it gets longer when there is more indentation.) I sometimes clean up these listgaps when I see them, and link the guideline in the editsummary to explain the cleanup and to gently inform/encourage others to try to avoid gaps. However blank lines are not uncommon, and attempting to enforce nicer-formatting would likely cause more hassles than benefit.