After looking around, my best bet would be either Off or On topic. If you are okay with unapproved members helping out, then you would have to give them a specific thread to chat inside of, since they cannot be given any offsite contact information nor can they PM. That would probably go in the Off topic forum.
If you just want it to be approved members, then it would go in the On Topic Forum, so that way you have a private way to talk and discuss it. You can also exchange private information there, like an email or whatnot, or you can always just discuss the blog in a thread and PM private information. It's all up to you.
I could be wrong, but I think your best bet would be On Topic. But it matters what staff thinks, not me.