The title of this question is ambiguous as I couldn't think of anything better. What I'm trying to articulate is the thought of 'I didn't know at the time but do now'. Can this be expressed by saying 'Я не знал', or does it go deeper than that?
For instance, after going up to someone and asking about their cat, your friend says:
Его кошка умерла.
To which you respond,
Я не знал. (Meaning I didn't know at the time, but now I do)
Yup, definitely works. It doesn't go much deeper than that.
If the situation is you specifically talking about some moment in the past, you may also add "тогда" or/and "ещё": "Я тогда ещё не знал, что ..." or "Тогда я не знал что" etc. However, it is exactly for "story" about past events, stressing the fact that at the time you describe you did not know something (which you obviously know now, otherwise you could not say anything about it).
