It seems likely to me that such areas are going to be ones that I habitually don't turn my real attention to, and that if they come briefly to mind it won't necessarily be obvious to me that I am treating them as blanks.

Yep, this is the key point: how to (better) notice what it is you're not noticing. So how do we drill down on this one?

This is the same key problem in working out what you really want.