Unfortunately, you can only go through Unova region in Black/white
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You cannot go to the Sinnoh region in Pokemon Black.
Sinnoh.
No but when you beat the main story if you have the sinnoh games you can do the POKETRANSFER on route 15 and send them over from that game.
No. You cannot go to any of the previous regions in this game.
The season that is after Pokémon Diamond and Pearl: Sinnoh League Victors is the season that is titled "Pokémon Black and White" which takes place in the Unova region.