This question is right up my alley, I was pro. dog groomer for 21 years and have cut at least a thousand nails if not more I'm sure.
With black nails take a sliver at a time off , when looking at the freshly cut area of nail when you see a black dot in the middle , stop. Now, sometimes the black dot may be dry looking , than you can go down shorter. The black dot is the quick. It's nice to have a product called quick stop on hand for when you accidently cut too short. but try not to cause it hurts them and of course can bleed all over place.