Due to it's current implementation, Blueprint lacks semantical document structure.
One step forward is T107213.
At time being, there's currently [[ http://www.w3.org/html/wg/drafts/html/master/semantics.html#outlines | no known implementation of document outlines ]] in graphical browsers or assistive technologies. Document outlines are not the way to go for now, but a clearer organization of headings and their sequence in HTML output would bring at least improvement to search engine indexing.
Therefore I'd propose to aim for:
# `<header class="navbar"><h1>`on [homepage] or `<h6>`on [subpages] [Wiki Title]`</h1||6></header>`
# `<main><h1 class="firstHeading">`firstHeading = Article Title `</h1>`
# Wikitext `<h2/>`-`<h6/>`
# `<nav id="side-menu">`
# `<h6>`[Wiki Title] in-context links `</h6>`
# `<div id="toc"><h6>`ToC `</h6>...`
Basically, every heading outside of `<main>` content (= most important document headings) will at maximum be marked up as `<h6/>`.