Dragon Black Market
The Dragon Black Market is a minigame which allows you to pick from a pack of six Dragons, five of which you can see, and one of which is a mystery. Based on this, you make the decision to purchase the list, at which point you are able to choose from six boxes. From this box, the game picks a dragon for you at random. If you don't like the one you get, you can choose another for 10 Gems. You always get the Special Dragon last and the Mystery Dragon before the Special Dragon.
See the Dragon Types and Breeding section for a full list of the Dragons in Dragon City - this will give you an indication of the types of Dragon that you can expect to find in the Dragon Black Market before you spend any of your Gems.