I wouldn't say charcoal is rare necessarily, but not too common either. Since it isn't standardized, it is hard to say one way or the other, and sometimes other colors (such as true blues) will get called charcoal too, especially if someone is describing what they see (a "charcoal" colored rat) and not what the standardized name is.

I wouldn't say they are chocolate either. Chocolate actually looks clearly brown, resembling a dark chocolate candy bar. Sometimes black can be faded or rusted to a brownish-black color, but true chocolate is rich, and even throughout, not faded or patchy. My experience with chocolate was they didn't rust very much either, whereas mink, blue, and even black (especially black carrying recessive dilutes) seemed to rust ("brown out") a lot.