Don’t worry it stands for Bootleg. Just found this linked to something at the excellent aggregator Elbows. It’s a blog dedicated to mash-ups and, as you probably correctly assumed, it’s got some good tracks, some bizarre mixes, and some absolutely atrocious shite on it. Most stuff is worth a listen, even if it’s just to find out what it sounds like to mash-up Biggie and Myley Cyrus (shite) or T.I and Tony Bennet (surprisingly ok/decent). Check it out here.