The negative lifestream is filled with those who died in despair. So much that their negative emotions makes it so they don't join with the lifestream proper, instead their essence joins the other. Geostigma expedited this process. Had it not been stopped, it would have turned the entire lifestream black.

[Though Cloud hadn't yet asked, now he gets to know that even Geostigma gets solved in the end. Sephiroth figures it's something he'd ultimately want to know.

His eyes stay on Cloud as he edges closer to the carving. Oh. Right. That.]

I believe it is the emblem you wear.

[There you go, Cloud. That's something of a joke.]