I had not known that before..
That is certainly possible in certain circumstances. For instance, you might say, "I hadn't known [or been aware of] that before I arrived in Tulsa. Had I known that earlier I wouldn't have bothered to come."
I have not known that before..
I can't think of a context in which this would be used. We would tend so say something like, "I didn't know that." "I wasn't aware of that."
We might, however, say, "Wow! A three-legged horse winning a race! I have not come across that before." The word "known" could not in this case be used in place of "come across."
I hope that is useful. I am glad you gave it to us to "chew on."