Tznius, I just want to clarify a few things about my earlier statements:
1. Everything I wrote and will write in this post is my own personal opinion. I never asked a Rav about black nail polish and never read anything about it anywhere.
2. When I said that it’s untznius, I did not mean that it’s assur. To my knowledge, it’s not assur. I merely meant that if you are asking about it, I would certainly say that it is a higher level of tznius not to wear it.
It is definitely not in the same category as wearing short skirts which is a clear-cut issur. I also would not put it in the same category as wearing certain styles which may not be mentioned in the Shulchan Aruch (since the Shulchan Aruch can’t deliniate every pritzusdik style that the oilam might come up with), but are CLEARLY assur to anyone who has a sense of tznius. I have seen girls wearing styles that were so pritzusdik even though they didn’t break any technical rules. I would not put black nailpolish in that category.
I would describe it as a level of tznius. It’s the type of thing that a Rebbetzin wouldn’t do and that one should be trying to outgrow, but right now, if you are a young girl and you feel that this is something that you like doing and that is hard for you to stop doing, it’s not assur, and there are probably more important things for you to work on whether in tznius or in other areas.
I don’t think it’s something to lose sleep over or worry about if it makes you feel good. Find something else to work on. At the same time, if you do want to send your kids to certain schools and be part of a certain society, you may want to think about eventually stopping to wear black nail polish at least by the time you have kids who are school-age, if not sooner.