- Visit a page in the Blueprint skin that has less than four section headings, e.g. Guiders page in the LSG.
- Open Blueprint's left nav "sidebar".
Result: you don't see the numbered section headings of the current page, so there's no special indication of your current place in the set of links in Blueprint's left nav.
Blueprint inserts the Table of Contents of the current page into sidebar navigation. It does this in toc.js simply by detaching the #toc div from the page flow and appending it to #side-menu .sidebar-toc-active. But the current page may not have a TOC for several reasons, including too few sections.
A partial fix is to force the TOC to be generated even if there are fewer headings. AFAICT there's no configuration for this, you have to change ( $numMatches >= 4 ) for the entire wiki in includes/parser/Parser.php. (Possibly Blueprint could hook into parsing and insert __FORCETOC__ in page text.)
Regardless Blueprint should probably indicate the current page in its left nav more directly by styling #side-menu .sidebar-toc-active, e.g. designs in T103719: Improve ToC or a simple ▷ indicator.