Black City is an area exclusive to Pokémon Black. Unlike it's White counterpart, White Forest, Black City focuses on battles and items, and wild Pokémon cannot be found here.
The city is very dark and ruled by greed, with trainers scattered around waiting to be battled. The Pokémon Center is in the east because the city has little light, and conserving electricity is important for the city. In the center is a market selling rare and expensive items, including lots of Evolution Stones. On your first visit here, the city may be tiny or something more impressive, this is because its appearance is based on how many characters live here. With a big population, the city has lots of magnificent tall buildings and a bustling market with multiple stalls. A lower population means fewer, smaller buildings, and a less active market.
The city grows as its population increases; you'll need to visit another player's White Forest via the Entralink to invite characters to your city and up your population. Not only do the characters affect the appearance of the city, they also determine which items can be bought at the market and the number of trainers available to battle. As well as coming into the city, characters will also leave after a while if you don't battle them often.
Inside the market, the boss will ask you to battle a certain number of people outside. Once achieved, you'll get a decent reward from him and he will ask you to complete another task.
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Black City has a maximum population of 45 (12 at the lowest)
It is stated that this city is ruled by greed, this is shown by the fact that most items sold in the market are extremely expensive.
All trainers in Black City have the fully evolved counterpart of their respective White forest Pokemon. For example, in Black City, Rosa has an Exploud, so if she was in White Forest, you'll be able to find Whismur in the wild.