An old post by Scott Alexander (16+ years, mind blown) discusses this, long before the term "aphantasia" became a thing [1]. There was a debate about what "imagination" actually means already in the late 1800s; some people were absolutely certain that it was just a metaphor and nobody actually "sees" things in their mind; others were vehement that mental images are just as real as those perceived with our eyes. The controversy was resolved by Francis Galton, who did some rigorous interviewing and showed that it really does vary a lot from person to person.
[1] https://www.lesswrong.com/posts/baTWMegR42PAsH9qJ/generalizi...
I actually feel like I'm closer than ever to getting towards visualisation. I've gone from a rock solid "zero" to "solid feeling, occasional split-second flash of something"
For most of the time with this exercise I was aiming for something simple. A red triangle in a blue square, but I'm not convinced that was an effective approach, I seem to be getting closer to the mark trying to picture something real.