I remember learning that by folding the thumbs under the other fingers to hide it from the light, you see the thumbnails in darkness while you see the light reflected in the other fingernails. This emphasizes the difference between light and darkness, during the b’racha of "borei me’oray ha’esh". True?

Correct. The common custom is to recite the blessing with the hand closed and the thumb in the shadow, and then to open the hand to place the thumb in light, signifying that we begin gaining beefit from the new light of the week at after praising Hashem for it's creation.