We can see black color when there’s no light at all. What I wonder is that there is concept spacetime even though there are no objects, but only color. I mean, does black color occupy the “space” (apart from mental space and real space) when there’s no light at all?
I assume you are asking this question from the point of view of a strict empiricist, and that by "concept of space" you really mean to refer to a concept and not the real space of physical realist philosophy.
If so, then the concept of space can certainly exist if there is just color. In fact, 3D games on computers are an example of this. When you play a 3D game, the only thing that is really on the monitor is colors, yet you see a space inside your computer screen. That space does not really exist, but there certainly is a concept of space.
The colors on your monitor are able to simulate texture and other visual aspects that present the illusion of space, so lot of depth perception comes just from color signals. The other main component of depth perception is binocular vision, which only works close up, and binocular vision requires color signals to let both eyes pick out the same point, so arguably, the concept of space comes from nothing but color.
I say "arguably", because your senses of touch and hearing also present a concept of space to the mind, and it is controversial whether or not those concepts of space are the same as the concept of space that you get from your eyes.
Another way to interpret your question is whether you could have a concept of space if all you could see were an undifferentiated black field. I haven't seen this specifically discussed in the literature, but I'd say in that case, you aren't seeing at all. It would just be a nonlocalized sensation like fear or thirst, so, no, in that case you would not receive a concept of space from your eyes.
The distinction between perception and reality is one of the important features that distinguishes modern philosophy from its medieval predecessors. The man who introduced this distinction to modern philosophy was Descartes who discussed the problems of knowing about the real world when there are dreams and hallucinations which seem to give the same information but are false.
The empiricists who followed Descartes were very focused on this distinction, so much so that readers today who do not have the same background often misunderstand them when they use words like "experience". To the early modern empiricists, an "experience" was a purely mental event. Whether it corresponded to something in the real world was always questionable. Today, when philosophers talk about "experience", they generally seem to assume that the experience is true--that what is experienced really happened. George Berkeley was one of the early modern empiricists who did not only doubt, but positively affirmed that the real world does not exist at all; only experiences exist.
This situation of worrying about the connection of experience to reality was a major influence in philosophy until Kant, who provided an excuse to stop worrying about it. Kant's philosophy was extremely controversial, but his method at least taught everyone that it doesn't matter all that much whether experiences are true or not. We can still function in the real world and still do science, if we only assume that our experiences of nature are reliable and rule-based in just the way that nature seems to be reliable and rule-based.
In other words, it doesn't matter if your desk is noumenally real. That is, it doesn't matter if your senses are reflecting a metaphysically real desk that is substantially like what you experience or not, what matters is that your sense of the desk is what it is, and will continue to be what it is. The metaphysical reality is irrelevant to action.
In modern terms, we might compare this to a Matrix-like situation. For purposes of functioning in the Matrix, it doesn't matter whether the desk you see is really a desk or just an experience created by electrodes in your brain. What matters is that it will work just like you expect it to.
In the physical world, the absence of light is just the absence of it, not a property called blackness or darkness.
Why is this different from experience?
Putting attention on sight and “seeing” black, or any other colors, actually means putting conscious attention on the output of the visual cortex, after the sights have undergone processing (edge detection, etc). We can do this even if it is entirely dark. That’s the key difference.
In pitch dark, the visual cortex will still respond with “heres what im seeing, basically black”, not with nothing. Thats why we have a concept of black instead of a concept of not seeing. If it had no report at all we wouldn’t experience it as black or dark; we wouldn’t even experience it as “Im not seeing anything.” We’d experience it as, “Im not seeing”