I didn't realize that, for example, "mental image" was not just, like, a metaphor for thinking. When I close my eyes, it's black. Just, black. Well, you know, depending on how bright the lights are, etc.
Maybe you already know this, but non-aphantastic (heh) just see black too. When we visualize something, it's a very different experience from seeing anything at all. It's more the knowledge of "if I could see this thing, this is how it would look".
I'd be very interested in knowing what really happens in our brains. It feels like stringing together a bunch of information and assembling a spectral "print preview" that you can't really see.
Then it might blow your mind further to know that I can visualize things easier with my eyes open. I overlay them onto my visual field and they're _far_ more vivid with my eyes open than shut, by an order of magnitude at least.
I am 100% jealous of your ability to do that, however.
I have recently learned that it might be possible to train myself to invoke mental images. There is a technique where a person with aphantasia sits their eyes shut and starts by imagining, e.g., a green apple. But you don't imagine it fully at first. You start with its different qualities, like imagine roundness for a while. And then greenness. Then shininess. Then some gradient. Then a stem. Then more detailed shape. Et cetera.
I have had a tiny bit of success with this technique, or at least I have fooled myself into thinking I have? If I try this, after a while I can start to picture a green apple, though it is like a faint, faded ancient photograph with almost no color or detail and certainly no vividness. But there does seem to be some little bit of something there.