You can get styptic *sp*?? pens which you hold to the toenail and it stops the bleeding. Also, a small little dish of flour and water that makes a paste can be applied to the toenails to stop the blood in a pinch.
I just take a little bit off each week because almost all of my dogs have black toenails
but I did clip one a little too close a few weeks ago and I was freaking out! If I didn't have the styptic pen I really would have freaked