You have misunderstood what the phrase "the dark side of the moon" describes. This is another way of describing the more appropriate term, "the far side of the moon." It does not refer to a side of the moon that is not illuminated by the sun but rather to the side that is "dark" or "unseen" from earth's point of view. The unilluminated half of the moon is illuminated from the earth's point of view by what is known as "earthshine"; that is, reflected light from the earth illuminating the part of the moon that is in shadows facing the earth. This is very visible when the moon is in its crescent phase, when the illuminated part isn't so bright as to obliterate the palely-illuminated part of the moon in shadows as seen from the earth. With this source of illumination and with the shape of the crescent moon, it is quite clear that the moon is a celestial orb. In addition, because the unaided eye can recognize certain forms on the moon, it is also clear that the far side of this orb never faces earth. It was then reasonably obvious to any observer of the heavens, regardless of the time in history (or prehistory) that there was a far side of the moon that was hidden from us.