It's never explicitly stated but it is implied. It has to do with how Ghosts revive you at all. Ghosts revive Guardians by grabbing hold of their Light, the essence of their being. Your Light can persist after death so that's why you can go hundreds of years before your Ghost revives you and be just fine. This might make you seem immortal but it's not. When your Light leaves your body it no longer has anything to protect it. Since Light and Darkness are antithetical they can not both exist simultaneously in the same place. They will eat away at each other until one of them is destroyed, usually whichever one there is less of. A "Darkness zone" is just that. A zone of Darkness. The implication is the hold of Darkness in these areas is strong enough to snuff your Light, and thus make reviving impossible, should your Light leave your body upon death. The reason as to why solo it's an instant wipe and why some times Guardians can revive each other can be interpreted as merely length of exposure or just game mechanics. The reasoning here is not as clear and is more open to speculation.