Many people are used to seeing grey/silver snails in the marine reef aquarium but are not used to seeing black saltwater snails in there tanks. Reef aquariums with live rock can sometimes have a whole slew of hitchhikers in them. Sometimes they are not seen for the first few weeks but they eventually come out, especially at night.
There can be hundreds if not thousands of different types of snails and for the casual reef keeper they can be very difficult to identify. The important part about identifying a saltwater snail is to look at its shell shape, its foot and its proboscis. Aside from the markings on the shell, the color of a snails shell has little impact on what type of snail it is. A couple of black saltwater snails that come to mind are of the nerite and stomalleta species. You can find more about snails on the right side of this page entitled “Beneficial reef aquarium snails”.