I understand your concerns, however we do have an ability and secondary typing to consider here. While you touched upon that subject earlier, I think Dark deserves it for the main typing a bit more than secondary, which is probably why I brought it up here and not later.
So because Dark has some merits and Steel has some merits, you pick Dark? Sorry if this sounds weird or harsh but I don't quite get what you are saying.
If anything, I don't believe that our main concern should be blocking immunities, rather it should be something with a typing that can fuck up the secondary. There are many ways to get immunities, but there are less ways to find a viable typing that can really scare users who do use the secondary.
Like I said, why does it matter if you can threaten Celebi and Rotom if you can't switch into their Secondary moves? (Not that Steel can any better, but since I am not saying to threaten Celebi and Rotom, I believe this argument hold more weight against the Dark typing.)
A problem that has occured in CAP 7 was that people were too focused on the CAP concept that they didn't even bother to consider how it would fare in the metagame that much, something that I think is happening here. To those of you who are vouching for a type just because of a certain immunity, I really do want you to think a bit harder as to what you resist and why it's important for this CAP. I urge you not to think simply immunities, but rather how to beat the secondary. Oh, and immunities isn't the answer I'm looking for. ;)
Again, Dark doesn't Stop The Secondary, only its users. If it can't do well against its users, I don't see how you are really fulfilling the concept.
Again, let me emphasize that I am aware that dark does not have the "cool" immunities or resistances other types may have, but I think fighting the secondary is more important at this time. We have another type to consider for this CAP, and the abilities to top it off. Don't get to close-minded here.
The same could be said for Steel. I already listed a bunch of good options to go with Steel.
Steel types can also use Pursuit; although non-STABbed, it resists Latias' and Celebi's main offensive moves as well as Rotom-A's Shadow Ball, which Dark-types can't switch in on as well, except Rotom-A's Shadow Ball.