The Black Market Auction House is a special AH that allows you to buy items that are rare and sometimes unobtainable with gold otherwise. It's an extremely effective gold sink, but it's tucked away in the middle of nowhere, and you can't see what's for sale on your realm from the remote AH. Unfortunately, since Blizzard doesn't make the BMAH accessible through the same API as the regular Auction Houses, you are expected to log in and travel to the Veiled Stair in order to see whether there was anything there for you.
The Undermine Journal is a website that collects, organizes, and displays information about the Auction House of every realm: Horde, Alliance, and Neutral. Additionally, starting in January, they launched a (still beta) feature that shows us the contents of the BMAH. Assuming you're not on a PvP realm, you can avoid the flight to the Veiled Stair to check whether there's something worth buying by simply checking the Undermine Journal.