Trimming black claws?

Look under the nail you will see a V shaped grove, as long as you don't cut past that you should be fine. Black nails are harder though. I personally use a dermal tool type grinder which makes it next to impossible to get to far down. (if you are grinding and begin to see a clear liquid you are getting close to the quick and need to stop. ) You can get them specifically made for nail grinding at the pet store or you can get a regular dermal and the sanding head (either the stone or the one with the replaceable sand paper covers)

On the underside of the nail you can see the quick. (after you clean out the mud or whatever else might be there) It looks like a black straw type thing. you can clip the nail up to that point. If you're not comfortable clipping them, ask your groomer or your vet to do it for you. (A good groomer would do that for you anyway.)

Just another tip: nails need to be trimmed at least every three weeks.

some nail trimmers have a guard so you dont cut too far down. If yours do not just cut a little at a time and keep baby powder on hand to clot the blood if you hit the fleshy part. After doing it once you should be able to do it again with ease.

The best thing to do is just trim the tips. PetSmart sells a new nail cutter that tells you where the quick is- I use this for my three dogs and it works great. I suggest buying one.