There is no Contest Hall in Pokemon Black because Pokemon Black does not have Pokemon Contests.
Instead of Pokemon Contests, Pokemon Black has Pokemon Musicals. Pokemon Musicals are held in the Pokemon Musical Building in Nimbasa City.
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If you win in the contest hall you get a ribbon but nothing else except for that
In Slateport City, the contest hall is close to the northern exit, just to the left of it.
Hearthome City
Contest Hall in Lilycove.
There is none. I know it sucks.