Jane has black nails, and when I got her they were growing kinda sideways and curly because she was at least four months old and they had never been cut. I did clip too far when I was trying to figure it out, but nothing serious. I have found that if you start with the longest one, and keep kinda shaving off the tiniest little bit, you will notice a change in the color, which will help you know when you are getting closer. Just remember that Rome wasn't built in a day, you have to work at cutting back black nails when the quick has grown too long.