A intentionally keeps B ignorant of C.
You could choose one of these (or some other word), depending on the situation. Keep on its own already supplies the meaning that something is done intentionally. Withhold and obscure are more specific:
I will keep it a secret.
Keep in the dark is a colloquialism that expresses more clearly what you mean in this situation. It means that you are keeping something a secret, but it places some unspoken judgment on whether or not that is the proper thing to do:
Alice: Didn't you tell him?
Bob: No, I'm keeping him in the dark for now.
== I'm keeping it a secret (although that might not be the proper thing to do) for now.
Here is your original sentence:
A's intentional keeping ignorant B of C is not morally acceptable.
You might translate this to using to keep in the dark in this way:
A's keeping B in the dark about C is not morally acceptable.
Or, more conversationally it might be expressed like this:
It's unacceptable that A is keeping B in the dark about C.
A is keeping B in the dark about C. That's unacceptable.
This sounds more formal than keep.
Do not withhold important information from the detective.
This means that you are intentionally making something hard to find or hard to understand. It may be an underhanded way of 'keeping' something from someone.
I wrote down my phone number, but I obscured the digits so that the clerk would have trouble making them out.