It's not just the blog - news.bbc.co.uk also intercepts middle clicks to external sites. Middle clicks on links to other bbc.co.uk pages work fine though.
I've noted this to them in their feedback forms before. It's been like that for over a few months. If I recall correctly when the Guardian (uk) was developing their current version under Beta the same problem came about for a short while.
I believe it's unintentional - I expect it stems from JavaScript event tracking (e.preventDefault(); logEvent(e); window.location=e.target.href; Or something to that effect).