This was until I tried some magic mushrooms (psilocybin as main ingredient) and I was shocked that with my eyes closed I was able to actually see things, mental imagery, objects, etc. After the effects wore off, the blackness has returned I couldn't see anything anymore. To me this is proof that aphantasia is real as I was able to compare it to a different state of mind where visualization is possible.

It's quite unfortunate that research on psychedelics is limited due to the insane war on drugs.