What do when you've completed Pokemon black?
You collect legendaries, find the sages, and train your pokemon.
Pokemon Pearl, Diamond, Heart Gold, Soul Silver and you can trade with Pokemon black/white. You have to complete the game and go to some lab with these scientists. ^ yeah, its called the PokeTransfer lab, but you cant get to it until youve beaten ghestis and stuff iits on the route after black city/white forest.
I don't think you can because Pokemon Diamond didn't have the new Pokemon that only appear in the Unova region. But, once you have completed the Pokemon League on Pokemon Black/White you can go to a science lab in route 15 and transfer Pokemon from Pokemon Diamond/Pearl, Pokemon Platinum, Pokemon Heart Gold/Soul Silver to your game.
Suppose you've just completed taking an online examination What happens the first time you click on Submit Exam?
You can only transfer Pokemon from Diamond to Pokemon black. You cannot transfer them back. Be careful. You can only use poke transfer once you have completed the main storyline of black and white. You need to go on the route between marvellous bridge and black city. And also, think about it. How is it even possible to put a Pokemon like victini onto diamond when he has not been introduced yet? *-*