The safest and easiest way to trim dark nails to prevent pain to your pet, is by shinning a UV light onto the nail to see where the quick is. The UV lights up the delicate bloodline, so you can cut in the right place.
Black nails can be tricky to cut, that's why it is important to look at the nail head-on, once you see a dark round circle this is as far as you should go.
 
At a glance:
UV Flash light for trimming dark nails, 
Detecting pet urine stains,
12 bright LED lights,
On/Off push button operation
Tough and water resistant casing.
 
Specifications:
Supplied with 3 regular AAA batteries
Long running time 5+hrs