i dont think theres anything about there being a dark sea dissapating in AA but the reason why the 6th sea was safe is a little complicated.
basically to get to the 7th sea you have to go east through an ENORMOUS stretch of empty water through the 6th sea and its INSANELY big.
this isnt the official map of what vetex had in mind for the 7 seas but its basically what it was meant to look like The Seven Seas
so basically after acherons attack this is what got destroyed (red circle)
