it means “could”.

I don’t know… that could be obvious at first.

The sentence is a bit complex here. I like your traduction and as a french person, i would have understood the same idea with your english sentence than the french sentence.
(Btw, IMO, the french sentence is bad)

Let’s change the sentence to make it easier to translate :

1) Ça pouvait ne pas être évident au premier abord

2) Cela pouvait ne pas être évident au premier abord

3) Cela n’était pas évident au premier abord

What we lost between 2) and 3) ? Pouvait . It means the possibility, the capability. It was possible that it was not obvious. But as you see the idea of possibility is not require (even in french). It adds no usefull information.

So, according to me, pouvait is useless here and should not be translate. But if you need to keep this information about the possibility, IMO, your translation is fine.

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.

The sentence is correct casual French, a formal form would have just replaced ça by cela.

I would translate it by:

I don’t know… It might not have been obvious at first sight.

but “at first glance* is fine too.

ManA wrongly thought ManB had broken up with someone else and is surprised because it wasn’t obvious that they were still together just by looking at them.

He is however softening his sentence by doubling the uncertainty of the non obviousness.