There is a color change in every nail. It is the difference between the live part of the nail and the dead. On black nails, one portion is flat black and the other gloss black, cut just shy of the color change. I usually grind nails because not only is there a blood vein but there is a nerve that runs down the nail. Sometimes the nerve reaches further than the vein and if you cut it, it is extremely painful to the dog. When you look at the bottom of the nail, the black spot is the vein and a white spot is the nerve. The nerve will not grind and the dog will pull back when you hit it. If you look at it right after then it looks like a piece of white fishing line. Last tip, the reason most dogs fight for nails is not because they don’t want them cut but because they are off balance when you pick up the foot. Find positions where you are supporting the leg so the dog is balanced. Resist the temptation to hold tighter when the dog bucks, rather release your grip and keep on just enough to allow your hand to follow the leg until they stop bucking. Less grip is more in a lot of cases.