This is all about your own design taste. Black crown molding with white walls can be a bold and classy look. It would be very modern and really give the room a distinct style. It would also go well with any accent colors you have in the room as well.
Black crown molding can be used to dramatic effect especially particulary in rooms with unusual or intricate ceiling lines. Pairing the molding with a lighter-colored wall lends a room a crisp and tailored appeal.
It is okay if that's what you like, however if you don't paint all the trim in the room black it will not look right. Also, if you decide to go back to white later it will take lots of coats of primer and paint to cover it up.