I have ptsd and bipolar disorder. I am on antipsychotics to stop having ptsd flashbacks but also inadvertantly cause aphantasia as well.

For what I've read, I'm not sure if Aphantasia can be acquired due to any form of trauma or (C)PTSD, there are too many variables to give a definitive answer on that. What I've read is that as you go through traumatic events or had a less than perfect upbringing, the mind can develop defensive mechanisms like you say which can start off as suppression and as you grow up, become automatic and turns into repression. That means you can "lose" the ability to do things people take for granted, such as feeling emotion, and possibly part of your visualisation ability if your attachment style developed into being Dismissive Avoidant, or even completely "block" memories and such when reliving a past trauma, or to prevent you from reliving a past trauma. Another thing I've read is that people's natural sense of smell, taste, hearing, interoception etc can be numbed or partially numbed when you live in a constant state of survival, as certain things are automatically deemed to be less "important" in that state than others. One of the things that apparently isn't uncommon to happen is when you go through therapy or do a lot of inner work to "rediscover" yourself, that those things I mentioned can return, or become more intense or profound than you know them to be. In particular, working on improving your interoception for what I read is one trigger that can make this happen. Does all this include acquired Aphantasia through trauma? I have no idea. But I personally wouldn't rule out the possibility without more concrete data specifically on the subject of Aphantasia. I believe Aphantasia was first discovered when someone had a brain injury and reported that they lost the ability to visualise afterwards, which quickly spiralled into other people with Aphantasia contacting the author of the news article, so that part of what you say is true.

As far as I can remember, I've always seen black when I close my eyes or when I dream, aside from occasional lucid dreams where I have limited control over the images I see. If you've experienced significant trauma in your life, aphantasia may be a blessing. My wife has hyperphantasia and is troubled by memories and dreams of past traumatic events that feel completely real (including sight, sound, smell, and touch (like wind on her face)). She has said she sometimes wishes that she had aphantasia like me, so she wouldn't keep reliving bad memories.