Personal experience – lemon juice is way too strong to use on your nails. Try it on just one – dip your finger in the lemon for a minute, then feel your nail. It’ll be all spongy and gross. Don’t do it. A better, and less strong, acid/bleach for fingernails is actually milk. It’s a good “chemical peel” for your face, too. Don’t leave it on facial skin for more than five minutes.
As far as your hair – dark hair doesn’t do much with lemon. Heck, it hardly does anything with my naturally blonde hair, except make it crunchy. Which is not pretty. At all. Chamomile+sunlight is better for lighting blonde hair. For dark hair the best thing to do is also the simplest – go outside! The sun will naturally, safely, and gently highlight your dark hair by bringing out the natural differences in your hair. My husband’s hair turns blonder. My friend’s hair turns redder. It’s really cool.