I'll assume you mean Xion by the black hair, in which case I'll call Oblivion relations, what with all the Memory basis and Snarl of Memories indicating she's related to Riku and not only to Sora
As for Namine, seeing how she was born when Roxas was, I'm personally blaming her pale skin and blonde hair on the same thing I blame Roxas's colors on - Ven.
The black hair was just an example.
But yes, I think so too, that the colors are because of Ventus.