Taldain is a tidally locked planet in a binary system where one of the suns is a weak white dwarf enclosed in a ring while the other a shiny blue-white supergiant. As a result in one hemisphere is always night-time and the other lives a perpetual day.
The star that faces the darkside emits lots of UV light, and the dark-side life has evolved to make use of this light: plants do photosynthesys with that light and it seems the inhabitants are mostly black skinned to get protection from that light and have fluorescent teeth and nails.
However, the day side should also receive lots of UV light and it seems only logical that the same environment pressure should make for a similar adaptation in life. I can see why the fluorescent nails are of no use, but why dark skin is not predominant in the day-side too? Would specially appreciate WOB (Word of Brandon).