I’m not a doctor and don’t play one on TV, but I learned this week on Royal Pains that the marks like those on the nails are indicative of ischemia and impending stroke.
In kids? I'm no doctor either but I am a parent of two. The older one's nails look to have blue paint or the remains of some nail polish on them. And the dark matter under the nails is simply dirt that hasn't been removed with a good scrubbing.
Doctors DO have a saying for this..."When you hear hoof beats, expect horses not zebras."