You'll have 6 dragon in a mystery box. Selection box will take a different amount of different gem. The number change gem seems to depend on the game itself. Also, there is a time limit that the black market will expire - that is, the game ends. Sometimes the dragon box will have a unique dragon that can only be obtained through the Black Market Dragon game.
How to beat the Black Market
First, you should not spend your gem to "Change list". If you're really looking for a specific rare dragon, chances are you will only see them after a few refresh the list. The best bet is to avoid using your entire options that just played often enough to always stay on top of the mini-game Black Market
Second, you should always have enough reserves gem to all 6 box open before the end time events. The Black Market Mini Dragon City game is designed so that you only get rare dragons in your 6th box. So, if you do not have enough gem to carry all 6 times, then you are simply throwing your gem in the normal dragon. Again, chances are unique dragon will be sold at a cheaper cost than gaining them from the Black Market.
source Gamesfree
