Ça pouvait ne pas être évident au premier abord.
Well, it is not an idiom, but it is not easy to understand at first glance ^^.
Let's go step by step.
1st Step
Ça n'était pas évident au premier abord.
Quite simple to understand and to translate :
It wasn't obvious at first glance.
2nd Step
Ça pouvait ne pas être ...
The key to understand your sentence is to understand the modal pouvoir.
Pouvoir translates the possibility in its broad sense : the right/conditions/probability to do something.
The following precise which modality is concerned. If something peut ne pas être or peut être, (regardless of the tense), that means the object is undefined, so the only modality you can use is the probability.
Keep in mind that the translation for maybe is peut-être.
Finally
The modal to use is may, affirmative mode.
It may
It might
Finally, use the first step gives the following of the sentence.
EDIT : past and negative mode.
... not have been obvious at first glance.