User blog comment:IamJakuhoRaikoben/Chapter 413: The Book of E.N.D, Review./@comment-4489126-20141222170143/@comment-9595671-20141223054643

The best explanation I can give for the blood thing is that they may have used the same magic that Doranbolt used to pose as Mest during the Tenrou arc: Memory Control. They likely changed their memory of the event, its likely they couldn't disappear on 7/7/X777 for unknown reasons.

Since Sting and Rogue were kids unlike most of Fairy Tail, the Memory Control magic worked better than in Mest's case. (Especially since Mystogon's own lack of presence exposed it)