I'd normally think the punchline is Sakuya "taking out the trash," but the tone and blush lines on Meiling's face don't fit that.

CrazyYanmega said:

I'd normally think the punchline is Sakuya "taking out the trash," but the tone and blush lines on Meiling's face don't fit that.

She spoke something that she shouldn't, I guess.

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.

translation bump