Some claim it was an apple, some claim a fig but from what I understand the bible doesn't mention what it was, its just called the forbidden fruit
Is this an example of not knowing so the blank is filled in by what we think or what it might be.
I wonder how much of the bible is/has been interpreted that way.
The Garden story in Genesis, contrary to Christian teaching, makes no mention of sin, original sin, the Fall of Man, death entering the world, spiritual death or anything like. Nor is the snake identified with Satan, nor does the snake tell any lies or practice any deceits. You only have to read it free of Christian encrustations to see that this is the case.
It seems to me that the story makes most sense if read as a linking story about the infancy of mankind between the creation story at the start of Genesis and the beginning of Hebrew folk history. Thus infancy is represented by the ignorance of Adam and Eve about good and evil.
And the tree of the knowledge of good and evil (with its fruit) is not to be found in any book of botany, because instead it's a metaphor for adolescence, the dawning of sexual awareness. Thus on eating the fruit they become aware for the first time that they're naked.
(The expulsion from the Garden then represents having to leave the protection of one's parents and earn one's own way. Note Genesis 3:22-3 where God states [his] reasons for the expulsion. Note in particular that those reasons have nothing to do with disobedience, sin, death or anything similar.)
It may instead have some folkloric meaning that's obscure now; but that's the best I can make of it. What is certain is that it's not about the fall of man or anything like that.