The Black Market Auction House (or "BMAH") is a type of Auction House that gives players access to bid on items normally not sold by NPCs. It has relocated to the west side of the Ring of Blood in northern Nagrand (it was located just southeast of the Tavern in the Mists at the Veiled Stair [63.8, 75.8]). The Black Market is run by the mysterious Madam Goya <Leader of the Black Market>.The plan is that the market will not be a safe zone, making it an extremely dangerous place for those playing on a PvP realm.
The items for auction vary by server, as other servers are able to bid on various items.
Bids start in the 10,000-20,000 range and there is no buyout option. The sky is the limit when competing against other collectors for items. Mounts seem to start at 20,000, armor at 10,000, companions/pets at 10,000, and misc items at 10,000 - 15,000.
The auctions last for more than 12 hours and return your gold to you when outbid or the item upon winning. After winning, another item is not added to the auction house immediately. The number of items that are up for auction is not the same across all servers.
Instead of buying items from other players like the regular Auction House, items on the Black Market are generated and listed by NPCs. The items are listed for one day only. The items for sale vary from items that were made unobtainable, to rare drops and TCG items. All items are listed infrequently, so it should not be seen as a reliable way to farm rarities.
Bidding works like the regular Auction House. There is no buyout price. The sky (or the gold cap) is the limit when competing with other players for the final bid. With a successful purchase, the item is sent in the mail. Normal Bind on Pickup rules apply, so you must bid on the correct character.
The supply of items is realm-based. Each realm will have different items for sale at any one time and prices are likely to vary wildly from server to server.
If empty, the item window will list "There are no items at this time. Please check back later."
These are items that may appear for sale. There may be more items which, due to the random nature of listings, have not been seen yet.
Most auctions are posted by various well known NPCs, depending on the item type:
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.
No PvP realms?
As I mentioned, the regular Auction Houses can be downloaded from Blizzard through an API. This is basically a feed that allows a program to request a complete copy of the entire AH. Part of the design of the BMAH seems to be having it a little bit farther away from the beaten path, though, and as such it's not included with that data. The Undermine Journal's solution to this is to use level one "reporter" characters parked in front of the BMAH on each PvE realm which log in once an hour to check the contents and report them to the site's database. Before Blizzard had this API auction feed, the Undermine Journal used to use a similar strategy to grab the regular Auction House data. An unfortunate side effect of this method, though, is that it doesn't work for PvP servers. It would seem that PvP players like to kill the level one reporters, and there's no easy way to get them back to their spot from the graveyard.
If you're on a PvE realm and stalking the Black Market Auction House for some of the high end PvP or heroic raiding gear, pets, mounts, or old transmog set pieces that pop up there, remember to bookmark your realm's Undermine Journal page. Alternately, you can follow the main Twitter account for the Undermine Journal. Every time the site sees a new item on the BMAH, it tweets it to the realm account (like @drenden, for me).