Blog is a bit of a difficult case because, as you point out, it encompasses multiple types of online site displaying attributes of blogishness.
There is a subset of Blogs that can be categorised as online periodicals - they fit the criteria to be allocated an issn.<http://www.issn.org/understanding-the-issn/assignment-rules/the-issn-for-electronic-media/>
Taking account of this, at the same time as withdrawing the proposal to make Blog a sub-type of Periodical, an example of how you would markup a Blog also as a Periodica<https://www.w3.org/wiki/WebSchemas/Periodicals,_Articles_and_Multi-volume_Works#Example_4_-_A_Blog_also_considered_a_Periodical>l was added to the proposal.
~Richard
On 12 Jun 2014, at 22:38, Aaron Bradley <aaranged@gmail.com<mailto:aaranged@gmail.com>> wrote:
"Periodical" is, by contrast, a less fluid concept. With perhaps some rare exceptions, a journal or magazine only exists as the sum of its constituent issues, and never as a physical object in itself.
FWIW I think Blog (the thing that contains blog posts) is an anachronism, and for schema.org<http://schema.org/> (like anything else where a definition ultimately rests on the mode of production, rather than attributes of the thing itself) is not a very useful type. And even less useful, and more confusing, is BlogPosting, which in every material respect is identical to Article (which is the only such object of this type - sensibly IMO - that you'll find in Open Graph). Ironically, most WordPress schema.org<http://schema.org/> plugins declare a blog post to be an Article. All of this to say that it makes sense that "blog" and "periodical" don't play well together, as a blog is such an ill-defined concept.