Blueprint.php hardcodes the set of pages that display in the Blueprint skin's nifty pop-up TOC. That's inflexible and prevents reuse outside of the #LivingStyleGuide.
Instead a message string should drive it, just like the sidebar in other skins. I think Blueprint can reuse PHP skin code like Skin->buildSidebar() or BaseTemplate->getSidebar() to parse a similar structure to MediaWiki:Sidebar.
Since Blueprint will only be in use in certain namespaces on mediawiki.org, it might be more flexible it to try the message key blueprint-namespacenum-sidebar before falling back to blueprint-sidebar.