This myth is half science, half easterm medicine myth, sprinkled with some christianity.
What makes the myth a bit convincing is that the part of the brain that deals with eyesight is at the back part of the brain at the back of the skull.
The old chinese medicine part is that when you keep a certain part of your body too cold, it is bad for this parts' health making it lose its' "chi".
Thus, the belief goes that keeping the back of the head cold will cause blindness (and too much heat can cause insanity, like when lying down right away after being active in the heat) But, as we know, this is not true.
This is likely a case of misunderstanding the biomechanics of how stroke occurs inside the brain, paired with Filipinos desperate and false sense of control, trying to avoid such health mishaps from happening to them. (And also Filipinos tendency to blame the person who is afflicted with the illness that 'it was their fault this happened to them' by twisting certain christian beliefs)