I have cut dog's nails for years now. Only once have I ever cut the quick and that was because the quick grew out to the end of the nail. With black nails the best thing to do is cut a little at a time if you are not sure how much to cut. It is harder to explain then show someone how to do it. Here is a link that tells you how to cut nails and it has great pictures and it is very detailed so you will know what you are doing and not second quessing yourself. It is a great link and if you know anyone who needs to know how to cut a dog's nails, it would be something you would want to pass on. Anyways here it is, enjoy!!!
http://www.vetmed.wsu.edu/ClientED/dog_nails.asp