Hmm...well, I don't think the color of the skin is limited to just one word. You can call it black skin, but most of the Silkies I see are ligher skinned than that. Alot of people refer to the skin as 'purple-blue' or vice versa. When I think of gypsy, I think of deep purple.
As far as the genetics of skin goes, I'm really not familiar with it. But putting it mildly, Silkies SHOULD have this purple skin, as in, accoarding to standard. I think the goal is to have it as dark as possible. Some have darker skin than others. It's all breeding and genetics. Different color variations can be called different things.