H1 in article page - site title or article title?

There is a W3C Quality Assurance tip about this topic:

<h1> is the HTML element for the first-level heading of a document:

• If the document is basically stand-alone, for example Things to See and Do in Geneva, the top-level heading is probably the same as the title.

• If it is part of a collection, for example a section on Dogs in a collection of pages about pets, then the top level heading should assume a certain amount of context; just write <h1>Dogs</h1> while the title should work in any context: Dogs - Your Guide to Pets.

As a screen reader user the heading level doesn't matter as long as it's consistent. Different blogs use different conventions, so there is no standard way to make it more accessible. Making sure the headings match to the content level is more important then what level of heading is used. For example if displaying a series of blog posts with comments all the blog posts could have heading level 2 and at the start of the comments you could have heading level 3. For an example of easy to navigate headings find any Wikipedia article with multiple sections and subsections. I can easily skip around main sections by using my screen readers navigate by heading feature to jump to any level 2 heading, and if I want to move to subsections of a given sections I will navigate to the next heading.

On your blog's home page, I would use H1 to denote the site title and H2 for the titles of the individual blog posts that are published on the front page.

When entering a particular article, though, I would use H1 for the article title and an anchor for the site title.

This is a nice and semantic setup that will also be appreciated by Google when it crawls your site.

On Wikipedia the h1-Tag contains the article-name and headings in the document start with h2. The name Wikipedia is part of the title-tag in the html-header. I also think that's the way to go. So for blogs I would do like Joel Spolsky in the examples you have given.

And I would always start with the highest level, so letting out h1 is in my opinion a bad option.