I think Sakuya's in her Halloween-form of some sorts as per Remilia's request. Meiling doesn't see any difference. Since Sakuya hints something being "black", I'd say it's about her underwear.
Meiling, this night I'm in my haloween moodHaloween mood ?Isn't it always the same thing ?The mistress wanted something different, so I had to respond positively Eh ?
I don't think there's anything diferent but...I would say it's always black