I did Aurora's nails just the other day and thankfully she only has one black toe nail on each paw so that made it simple. I keep cornstarch or flour near by in a small cup. Working with a vet previously having a small cup near by was much easier to use as you place the dogs paw in the small cup rather than trying to put the flour/cornstarch/quik stop on the nail.
Thankfully I haven't nipped Aurora's quick, nor any previous pets, but I've seen a vet do it!
Years ago when we had a large breed with black nails the vet clipped him too far after his neuter, and he didn't start bleeding until he was in the car on the way home!! Man can the quick bleed!!