Because I'm not distracted by mental images popping up, its only a function of slowly watching your thoughts pass by in complete blackness (normal for me). When talking to fellow meditators without aphantasia I feel it makes it easier for me (once I learned not to control those images) to enter those stages quicker and more reliably by taking a shortcut through the realm of visual blackness ;)
Since I don't know how it is to be able to conjure up mental images at will in the ordinary state of mind I tend to be more verbose and exact in describing my experiences to others who in turn can be more easily triggered by that - I suspect through the help of their mental images now popping up. I find it fascinating that I can literally evoke elaborate pictures in most people minds by just talking to them with attention to more detail (which I can only abstractly conceptualize).