Yes, though I don't know could be replaced with not being aware, though whether or not it is a state of mind is dubious because can something that you're not aware be a state of mind when it does not even exist in your mind? But yes, if you're completely unsure of something you can say that you don't know, as your knowledge isn't great enough for you to lean to belief or disbelief.
So basically you are saying that there is one more option than being for or against something. That's the middle ground that can be defined as undecided or unaware.
For example, if I am not aware of Nem's argument in this thread, you are claiming that I would be neither for or against it. But once I read it, I could be either undecided or against it.