Phantasmal Killer, PHB p265
You tap into the nightmares of a creature you can see within range and create an illusory manifestation of its deepest fears, visible only to that creature. The target must make a Wisdom saving throw. On a failed save, the target becomes frightened for the duration. At the start of each of the target’s turns before the spell ends, the target must succeed on a Wisdom saving throw or take 4d10 psychic damage. On a successful save, the spell ends.
The wording in the description uses visible, i.e. only the target can see it.
It appears that the simple RAW interpretation of this is that if the target is unable to see the illusion when the spell is cast upon them and continues to be unable to see the illusion for the duration, it has no effect on them. The spell says nothing about it being anything but visual illusion.
Is this interpretation, RAW, correct?
This hugely depowers the spell as it won't work in the dark, if they are blind or even blindfolded.
I have asked an important associated question here: Should Phantasmal Killer work in the same way as Phantasmal Force