I can’t help you with the horn button but some have suggested the same reason for the that the black roundel design (to memorialize Jim Clark). I have read that that is mistaken but in a quick search, I haven’t found a good discussion of it.
I don’t really keep very close track of this sort of thing but my TCS came with the green and yellow roundel and I believe I’ve seen black roundels on an Esprit. The steering wheel hub on my car has a black emblem as do Elans. As most of us know, my memory deserves a pretty short leash, but I’m not sure I’ve ever seen a steering wheel with anything other than a black hub or horn button.