Most Helpful Girl
I've painted my nails black before for several reasons:
1. part of a fancy dress costume
2. it matched my outfit
3. I was at my friend's place and we decided to paint our nails
4. I felt like it
There is no one size fits all. I'm pretty sure there will be some girls out there who paint their nails black to try and convey their emotional state, to feel like they're rebelling, to impress a boy/girl/group of people, or to convey a certain image of themselves.
But then again, maybe it's a mark of a super secret girl society that nobody talks about...