Maybe this site will help with those black nails, although I have to say that I've never seen anything look like this on the tip of my dogs' nails. Scroll toward the bottom of this link:
I've read other places that say there's a groove on the underside of the nail that shows where the quick is. Sorry, maybe my eyesight is bad or the dogs here are too small, but I've never seen this either. Well, maybe on my German Shepherd I had as a kid. She was the only dog that ever had those hooks that are pictured too.
Another site describes more what I see on my dogs with the black nails. The outer covering of the nail is black, but clip off the end and the inside is first white, and then keeping nipping and the middle will start to have a gradually increasing dark/black spot in the middle of the tip as I nip it shorter, and then when I see a whitish or pink dot in the center, that's when to stop because that is the quick. It might look moist too. This was how my old vet described it to me, and she drew 4 pictures similar to the one below, and said to stop before getting to the 4th one. Note - my picture isn't intended to show the pink in the middle, only the size of the black dot increasing. The lightness toward the middle of the dot is only from the ink pen not covering evenly:
Here's a site with a video where the tech describes this black dot too. Fwiw, only Ben gets hooks on the ends of his nails, the girls nails just keep growing out with a gentle curve but no hook, ever: http://www.ehow.com/video_4755667_poodl ... continuous
So Kara, what say you about the black dot theory?