My rabbit has dark brown nails too, so I was initially hesitant to trim them because I was frightened I would hurt her. I found a good trick is the "squeeze test". Position the clippers on her nail and squeezethem twice (but don't actually cut through the nail). If you get a reaction, then you are too far down the nail and need to reposition the clippers. If you don't get a reaction, then it is safe to cut.