When I "imagine" images, they're not usually superimposed or mixed into the input from my eyes like you described. They're on like a "second computer monitor" in my head. Maybe this helps you understand what it's like?
I "see" what I imagine, but I also can see what it is that's in front of my eyes, and the two things don't conflict with each other.
Here is another description of it, and a test in the first part: https://www.youtube.com/watch?v=ewsGmhAjjjI
I mean, how does driving work, when you see what you imagine?
Edit: And now that I think of it, I have had some accidents where I reacted to what I expected the other driver to do. But they did something else. One in particular I remember. The car ahead of me had a turn signal flashing. I thought that it was the right one. Or at least, that's what I remembered. So I passed on the left. And hit the other car, because it had turned left. And I recall thinking that I actually saw the car turn right.
I'm glad that doesn't happen more often :)
It does remind me of blindsight. That is, people who complain of blindess, but obviously do react to visual input.
From wikipedia "spatial imagery consists of mentally representing spatial relations between the parts or locations of the objects or movements.".
Analysing whether a written description of fight choreography would work in actual 3-dimensional space is very much spatial imagery, a strenght of aphantastic people.
I can also reason about the three-dimensional shape of a banana and create a technical drawing of one, rotate objects in my mind, fold objects in my mind etc. But all this sounds like spatial imagery, and does not involve me actually having a mental image.
I wouldn't describe it as "mental images", but I certainly have mental representations. But these mental representations are about the 3d shape, the behavior, texture etc. Images on the other hand are not properties of an object, they are projections of 3d geometry into 2d space (with the added work of simulating light to properly shade the image). I suppose I would be decent at painting if I put the practice in it, but the idea of an actual mental image is foreign to me. For me, creating an image is basically a conscious translation task, and without a medium to record my progress I can't handle at best basic shapes.
Basically talking about images is no different to me than talking about textures, or the propagation of smells, or the orbits of planets.
But anyway. Normally, I do not see stuff that isn't (arguably) there. With my eyes open, or closed. But I can imagine stuff, and reason based on those imagined images or whatever. That is, I can do all those object rotation tests. I can imagine some thing, and create it in wood or metal, without anything but crude drawings.
Or when I read a good description of a fight, for example, I can imagine how it would work. Or not, if it wouldn't.
But is this basically a form of blindsight?
Also, it was my impression that people who experience stuff that isn't there are typically diagnosed with some sort of psychosis. And from my experience with psychedelics, I know what that's like. Even with good marijuana, I sometimes see and hear people talking to me, just before I pass out.
I can't imagine how it would work if I were constantly seeing stuff that isn't there. How could I, for example, drive a car? Or walk across the room, even?
So am I confused, or aphantasic?