Maybe the Black Queen is a possessive girlfriend? "I'll do the march of the Black Queen" = "I'll obey her, 'cause she's nuts"?? Dunno, that's just how I interpreted it. But I do agree with the White Queen contrast. White Queen is a very soft, loveing song while Black Queen is harder. Interesting how the band put subtle things like that into their albums.
I imagine the White Queen as a maid dressed in white and perhaps with dark hair (in contrast with her clothing, announcing her future) and the Black Queen as a totalitarian ruler in a country (or kingdom, whatever) with no women.
I can see her related with the Killer Queen too (perhaps a middle stage).
Maybe a queen from an african country
White Queen must be the dutch queen, you don't want to see here in an bathing suite
She must be white like milk
Michel