concise word or phrase to express the idea that one does not make a judgement due to lack of information
Solution 1:
It's not easy to find a suitable phrase, but you could consider saying,
I can't make an informed decision (now), but my initial response/idea/suggestion is that...
Informed in the sentence means:
(Of a decision or judgement) based on an understanding of the facts of the situation: she lacks the ability to make an informed decision.
[Oxford Online Dictionary]
However, you can never know the reason for your not being able to make an informed decision is lack of information available (more likely) or lack of your knowledge to understand it. It will depend on your context.
Solution 2:
"defer judgment", "suspend judgment" are not all that much different from "reserve judgment" but possibly less definite on the eventual resumption.
"refrain from judgment" is definitely something that can end up as permanent postponement.
Solution 3:
I’m reluctant to answer that question [now] because I’m simply/frankly not [yet] up to speed on that [topic].
up to speed
- …
- Informal Fully informed; conversant: I'm not up to speed on these issues yet.
(from American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2011 via ‘The Free Dictionary by Farlex)
The bracketed [now] and [yet] would make it clear that the intention is to eventually answer once the speaker gets/is brought ‘up to speed.’
It’s arguable, however, that using ‘not up to speed,’ even without ‘yet,’ might give the impression that one will eventually make the effort to get there, which would admittedly render it more suitable for your secondary inquiry.
If that is the case, perhaps the addition of ‘simply’ or ‘frankly’ would help convey the “don’t know, don’t care" notion of your primary use/inquiry.
Here's an example of usage from ‘From the Bureau to the Boardroom: 30 Management Lessons from the FBI’ by Dan CARRISON via Google Books.