Is it foolish for a blind man to believe there is no such thing as light?

A blind person would have no way of recognizing it. S/he would feel the sun's heat but not necessarily know that it is light. However, if the blind person were raised in a loving home, and the child trusted its parents, it would be able to believe that light exists, even if s/he can't conceive of it. the child could learn its properties

Yes, because he knows that he's blind and therefore cannot register the light spectrum. There are biological and physical reasons that he cannot register something that has been empirically shown to exist. This is NOT the case with the thing you're trying to make a point about, so the metaphor is flawed.

Is this under the assumption that the pet son had been blind from birth or a very young age to the point s/he doesn't remember light? Or, had the person become blind at an older age so that s/he remembers light?

no, but light is only a particular range of wave lengths in the electromagnetic spectrum and is a definition given to it purely because we can see it, therefore light does not exist any more than red does to a colour blind person. microwaves are part of the same spectrum and I can't see them, so I an blind but I can carry out tests to prove they exist and predict their effects, leading to the microwave oven

that's science

we can all see at the psychic level and we need to emphasise this in all of our lives and then the blind shall see Him in all of His glory for Christ is today on this blessed earth to redemm all of our souls eventually, so come forth blind men and see within the third eeye and all shall become as light. The third eye is now opening up naturally for quite a large number of folks today, it is part of our ongoing evolution to see sound and hear light as we say in the occult sense.

when true light comes it can be seen in the head centre itself. Saints are said to have this faculty of seeing other realms of light and beings etc,

Om Shantih.

Isn't it strange how they all turn to natural reasoning, when it comes to spiritual blindness and vision (on a religious forum)?

To answer your question, YES! We were also spiritually blind before we could see the LIGHT!

I can't see ultraviolet light, but it exists. We can test for it, and have it independently confirmned. We can also observe it's effects (even the blind man could do this). So even though there's parts of it that aren't part of his personal experience, he can still be given evidence of it.

Many of spectra of light are invisible to us but we know they exist by other means, that's a flawed metaphor you are using there

Is a reliable person saying that there is light? And they've seen it?

Then it probably is believable

If I can feel the warmth of the sun and the coolness of night I can understand the two companions. One does not need to see to "believe" but to actually understand they do.