A constantly-evolving black metropolis that only Pokemon Black players will see. As time goes by, the population of the city changes and its appearance as a consequence.
Exclusive to Pokemon Black , Black City is an unique area where the contents of the city depend on your actions.
In particular, the number of residents varies according to how often you visit Black City and how often you interact with the residents. By connection, the number of residents influences the landscape of the city: the busier the city, the more buildings there will be.
Some of these residents will be Pokemon trainers, who you can battle daily; great for building up your Pokemon party or amassing funds. If there are enough trainers in town, a market will open, offering a wealth of rare items. A win, win situation, basically.
However, if you neglect to visit Black City and/or interact with the trainers, some of them will pack their bags and leave. Similarly, if you took ages to reach Black City (since starting the game), you may find yourself with a deserted Black City...
To bump up your population, you will need to find somebody with Pokemon White, then use the Entralink to access their White Forest , where you can invite residents to your Black City (don't worry, they will stay remain in your friend's game!).
If no such friend exists, well... that sucks, but you can find Pokemon battles and rare items elsewhere.