Personally I think that you should wear whatever you like and whatever you're comfortable wearing. If you like it, don't worry about what others think.
There is one exception however. For the purposes of magic, you're goal is to amaze people. Now, depending on your image, and the type of person you are and magic you perform, black nail polish may or may not augment your magic. So for personal use, go ahead if you want - but do consider what effect it has on your magic and your presentation, because it does give you an edgier, more alternative and possibly even confronting look.