I would like to cut my dogs toe nails, but since there nails are black I can't tell where to stop.

If your afraid you'll hurt your dog ask your vet to help you or give you tips or tricks

There are also nail clippers for dogs that have a guard on one side. It only allows you to clip the so far up from the end of the nail. You need to be careful with that though, because if you're already starting with a nail on the shorter side the guard may not help much.

You could always do what show dog people do, They use grinders. I believe they sell them just for the purpose of doing nails. Check at a pet store or online.