Thank you. Is there any meaning to saying that even though it is an absence, it could still be called created? That is, could the very possibility of emptiness, of absence, itself be a creation?

Maybe.

Rabbi Saadia Gaon writes this in the clearest possible way:

Book of Beliefs and Opinions, First Treatise, section 3:

"Darkness is not an opposing element to light; rather, it is the absence of light. And how do I know that darkness is not an opposing element to light? From three proofs. The first is that a person cannot create an element, yet we see that when he stands in the sun and places one palm over the other so that there is shade between them, it becomes dark there. Thus, the person did not create the element of darkness, but merely concealed the light from the air between his two palms, and therefore it became dark because the light was absent. The second is that I see that a person has a shadow when he stands before a single candle, but when we surround him with many candles he has no shadow. Surely a person has no power to destroy one of the elements; rather, he supplied the light that had been absent from part of the air surrounding the person. And the third is that I have never seen two opposing bodies where one turns into the other in complete form, just as water does not turn into fire nor fire into water. So when I saw dark air become illuminated, I knew that darkness is not something opposed to it…
And I know that God already described Himself as 'forming light and creating darkness,' and He says this in a way that corresponds to this observable reality: that He created the air, which receives light and darkness through presence and absence. It is similar to what He says afterward, 'making peace and creating evil.' And we agree that the Wise One does not create evil; rather, He created things through which there can be peace and evil for a person according to his choice. For if he eats food in the amount he needs and drinks water in the amount he needs, this will be peace; but if he takes more than he can bear, this will be evil, as we will explain in the fourth treatise regarding justice. But light and darkness are attributed to Him as His creation in opposition to the wicked claim of dualism; therefore He said, 'forming light and creating darkness,' and He further informed us that light and darkness have purpose and limit."

And plainly, this is also the meaning of what is written in the Book Bahir, chapter 1, section 13:

"Rabbi Bun sat and expounded: What is the meaning of the verse, 'forming light and creating darkness'? Rather, concerning light, which has substance, the term 'forming' is written; concerning darkness, which has no substance, the term 'creating' is written, as it says, 'forming mountains and creating wind.' And if you wish, say instead: light, which has actual coming-into-being, as it is written, 'And God said: Let there be light'—and coming-into-being is only through making—therefore it is called formation; darkness, in which there was no making but only separation and distinction, is called creation, as in the expression, 'so-and-so recovered.'"

With all due respect, darkness is not the absence of light, but something pushed back by light.
I’ll explain.
Maimonides, in Guide for the Perplexed, part two, chapter 30, writes that the element that produces darkness is fire, and fire in its original state is black and cold; it is only that God clothed the fire visible to our eyes with heat and light.
And Maimonides continues and says that if the element of fire itself were burning, all the space outside the globe of the earth would be burning all night and all day. A somewhat frightening sight, in my opinion, more than empty space.
So darkness rules everywhere. But by means of light we push it into a corner, a kind of contraction of its area, yet still in the corners and edges that the light does not reach, even though the room is lit, there is black fire there, which is really the darkness. And when we turn off the light, the darkness returns to its place.
Maimonides there proves this from explicit verses.
So darkness is basically a creation that can exist in the absence of light, and when there is light then the darkness is pushed aside.
I also found more material on the subject at the following link:
https://chabadpedia.co.il/index.php?title=%D7%97%D7%95%D7%A9%D7%9A&mobileaction=toggle_view_desktop

So there is an explicit Maimonides saying that darkness has substance. QED. 🙂

So if darkness is "black fire" and someone (God?) decides to extinguish it, what is created underneath it? Fire that is not black? Or is black fire eternal by definition, so that there is a logical impossibility in extinguishing it?
And another related question that comes to mind: does stupidity have positive existence? Some sort of "black stupidity" that threatens wisdom (the non-black kind, of course)? Does the Chabad site have an answer for that too?

Science too, in the Big Bang theory, which is accepted by the overwhelming majority of scientists of all kinds, claims that there is "dark matter" that performs one action or another, without which the universe could not exist. So I would not call this thing "stupidity," but by another name that Maimonides gave to this matter. And what can you do—Maimonides did not know all the definitions of the Academy of the Hebrew Language, since he lived about 850 years ago.
And before attacking head-on, and certainly things that Maimonides wrote, I would take a little time to think.
You can search on Google what dark matter is. Then you’ll understand that even though Maimonides lived before the invention of one technology or another, he knew "a little" more than we do.
And regarding darkness, it definitely has substance. Heat works through disorder of the atoms, and cold is the opposite. Therefore it is easier to heat than to cool (microwave versus refrigerator—what does the job faster?), and in space the average temperature is… 270 degrees Celsius below zero. That is to say, an extreme ordering of atoms.
And yet there is darkness there? The necessary conclusion is that if darkness is cold (and ordered in atoms [thermodynamics]), that means it has substance.
And let’s continue: if cold is an element of darkness, then heat is an element of light. And just as cold is not the absence of heat but order in atoms, so darkness is not the absence of light but order in particles. And light is the disordered one, just like heat.
P.S. As for the mass caused by dark matter (because why would matter that is in empty space with zero gravity orbit around something?), Aristotle and Plato already called them "the sphere."

I didn’t understand. Is darkness—which in your view is a physical entity (or at least has a measurable physical magnitude)—the product of…? If I removed it, not by means of a beam of light, what would stand "behind" it?

That’s the point: as with heat and cold, if I managed to remove both cold and heat completely, what would stand behind them? That is a question with no answer, since only these two states exist in our universe. Even if we feel "comfortable," meaning neither hot nor cold, that still does not mean there is no heat and cold, but rather some particular measure of both together.
If darkness really is the absence of light, then blind people should be seeing darkness. Because all that one sees is light, and they cannot see it, so they see darkness—that is, nothing.
And that is not true. Because blind people do not see darkness, just as our kidneys do not see what goes on inside them. And although there is room to philosophize about this, a blind person does not see darkness, because what we define as darkness is due to light, just as the definition of cold is due to heat. And someone blind from birth has never seen light, so he does not know what darkness is. So what does he see? What does he think about?
In my opinion, the answer to this is like a parable of a two-dimensional world. If we take a book and show it to a person with two-dimensional vision, from one side he will see the front cover—broad and long. From another side he will see the side edge of the book horizontally—long and narrow, and from a third side he will see the bottom edge of the book—narrow and short. And if he asks, of everything he has seen, what defines a book? The answer is: all of it, even though from his perspective that does not make sense. But that does not mean it is not true.
So what would happen if there were no darkness and no light?
Maybe whatever would happen if there were no heat and no cold, or when we understand what a blind person actually sees.
In short, from this perspective we live in a two-dimensional world: heat–cold, light–darkness, seeing–and not seeing, hearing—and not hearing. And what is in the middle? We do not know, and probably in the near future we will not know. But the fact that we do not know does not force what people want to think.