One thing we have to remember is that with the typing Grass Dark we almost certainly will be threatened by a variety of types. Being weak to Fire, Flying, Fighting, Ice, and Poison with a 4x weakness to Bug means that no matter what we do our mon will have lots of Problems switching in. Factor that in with the fact any Pokemon will U-turn will basically get a free turn out of whatever you switch in against it.
In other words, We will almost always loose the switch war without previously gained momentum.
So, who can a Grass Dark CAP battler gain momentum on, strictly based on typing?
Psychic heavy hitters (there's a lot of these!)
Bulky steel types like ferrothorn, forretress (He doesn't learn U-turn), and metagross
Who can't he?
Anyone with U-turn, period. This includes Landorous, Scizor, Infernape, Mienshao, Celebi, and occasionally Jirachi, Gliscor, and Hydregion.
Volcarona eats this typing alive.
Every single dragon type in OU has a standard set that has a super effective move against it. This came be overcome with high defenses, but that would be both mediocre because of it's typing and somewhat useless because trying to counter every type it's weak against would be a waste of time and a waste of effort. It's Supposed to be weak to certain things. We'll just have to see how this turns out.
More stuff to consider. Important to know what makes sense just because it's typing is so restrictive.
A choice banded critical V-create from darmantian with a +6 boost to attack, sunshine, and an adamant nature on a level 1, dry skin, mild paras with a -6 boost to defence and 0 IVs in defence will do 16648780 - 19586808 Damage.