People have different contexts. Usually, from what I've seen, people do have well-developed and shaded and colored views of the world, but oftentimes see different colors, so to speak. You only see the black and the white of their metaphorical art, and miss the shades ultraviolet and infrared that are totally outside your experience. Upon switching contexts one can see that the ultraviolet and infrared are oftentimes not only adequate for handling the shades of gray, but are in fact far more apt than one's own frame of reference that limits things to mere shades of gray.
It's entirely possible to have two very smart, very open-minded people accuse each other of black-and-white thinking, if their contexts are different enough. Artists might see math as drab and limited and boring, as compared to the wide world of unrestricted imagination they paint on canvas. Yet the mathematician uses an unrestricted pallet of different kinds of numbers to creatively paint on a canvas of logic, all the while perhaps seeing the artist's world as drab and boring and pretentious.
In general, there is no "ultimate context" to which we may practically subscribe in order to be "really openminded" and thus confidently say others have lesser contexts that engage in black and white thinking.
My apologies to the OP if this diverges too far off topic (I kind of went into meta-topic mode). I think it's a brilliant idea for a thread, and I'm glad you made it.