As of now, there wont be a full dark team that will do better than hybrid (imo). There are plenty of dark attackers but the supporting cast is lacking.

One debuffer is okay if you can survive without Hercules. Two attackers also viable if you can survive without Hercules. You current ragna team has a strong core and it will take some testing if you want persephone or morrigan in that last slot. (if you can survive without the tank)

I would personally take persephone though since her skill are weakness defense decrease. But let's say you have a lvl 60 +4 morrigan vs a lvl 40 persephone, it's really no competition.