oxidized platinum

you can transfer Pokemon from Pokemon platinum to black and white but you can't trade from black to platinum

No, but you can import Platinum Pokemon to Black.

Platinum ring NEVER turns black.

Only sliver rings turn black.

You can never trade Pokemon from Black to Platinum, because Generation V Pokemon do not appear in Platinum or any other game before Black and White.

However, you an transfer Pokemon from Platinum to Black after you beat the game.