CAARK!
Black and White thinking can be bad, yes. It can be used to pass over the suffering of others with a blind eye, or to justify cruelties beyond imagining as long as the Other is considered to be wicked and wrong in all ways.
But things are never that simple. Just as the world is more nuanced and mixed up, full of exceptions and wrong impressions, filled with countless other points of view that would grant a mind breaking number of different colored lenses from which one could understand this farce we call reality, so too is even the idea of black-and-white as a narrative device much more layered than most people seem to notice or care to see.
To be clear, we're talking about the kind of thinking that draws a clear division or difference between two things. We can actually break this up into different literary devices in writing. The first and most obvious is Juxtaposition.
Juxtaposition is when you place two different story elements side by side in order to compare how similar they are in some respects, but also to highlight how contrasting they are, how different.
But less pointed out is how this Juxtaposition can be a form of Metonymy and Motif. These two are connected. Motif is a symbol, structure, or idea repeated throughout a literary work to emphasize the theme. Metonymy is the deliberate exaggeration of symbols so that a short phrase may represent a much longer concept ("The Pen is Mightier than the Sword" is a good example where the pen represents for writing, speech, and other communicative forms used to foster peace and cooperation, while the sword represents physical force and violence to solve problems). This is often used for dramatic effect, as your teacher has said, but it can also be to make a point or clarify a subtextual idea.
This method of using juxtaposed imagery, ideas, and even characters to convey much deeper layers of subtext is not a new phenomenon. It's very pervasive in literature. Have you ever wondered why Darkness and Light are used to refer to Evil and Good? Actual darkness isn't evil, there are many things that depend on it for survival, it's a natural function of life. But by equating it metaphorically as a symbol of an abstract concept such as Evil, they suddenly have imagery they can use to clue the audience in without explicitly pointing out how evil their enemy is every time they appear in a scene.
And even more pertinent. Not only can this juxtaposition and symbolism be used to emphasize things such as differences and themes and concepts the audience should be taking note of to understand the narrative, but it can also be used to hide things or to draw attention away from them.
Take many old stories that depict armies against one another. There's the "Good" army whose side we are hearing from. And the "Bad" army who is the enemy. Today we tend to focus on the idea that "The Victors write History" and while i'm sure that's very true, it's missing the point entirely when it comes to much of literature.
Anyone who has been in a battle, or who have soared above it as I have while looking for dead flesh to feast upon, knows that things are not actually black and white. There are good people on both sides of any conflict, and bad on both sides. There are fools and the wise. There are abusers, there are healers, there are protectors, there are the meek, the craven cowards, the pacifists, the self-righteous, the wrathful.
People cannot be divided upon a line between good and evil. Not in any real way. But consider, for a moment, what it means to even try to depict that in a full narrative of any kind. The sheer amount of detail and time necessary to do the reality any justice would be staggering. Not impossible, but it would in many cases be so overwhelming to the narrative that any theme or idea the teller meant to convey or talk about would be drowned in a sea of minutiae.
To tell a story is to convey at least one idea across to the audience. Often even multiple for the experienced storytellers, though it can be a lot to juggle at one time. But while stories contain multitudes of symbols already, they usually are set to work in relating to or bolstering at least one main theme or concept that fills the whole of the tale. When you have too much symbolism, too many ideas, the themes and ideas of the story are buried beneath the waves.
So you see, like many things, this sort of black and white storytelling can have its uses, and it can cause harm when used without thought. In the world of narrative and literature it has its place, and you must learn how to recognize it whenever it appears and understand its purpose and effect in each different work.