Rarity noticed the effect happening but she made the - possibly - wise decision to not make a remark on it. She didn't wanted to receive more unpleasant surprises. The story that was told by Luna was still being processed within the mind of the mare. The vampiric unicorn tried to wrap her head around it but she didn't seem to be able to. That too was the reason of why she didn't dare to ask further. No further explanation would have been asked, because she didn't dare to.