I know there are a few odd black face plates and covers on some models. I have noticed it on machines made around WWII, and I think there are a few earlier and later examples too. I don't know the details, but some are particuarly looking for black side Featherweights and are willing to pay for it. I have seen dark presser feet and accessories, black iron oxide finish in stead of the shiny chromium. Early singers had nickle plated shiny parts, but some time after 1920 chromium became the standard. The general info on this is that Singer turned to alternatives when chome either became hard to get hold of or was seen as and unnecessary an excess in those days. I think it was a fancy too, since they turn up later when chrome was more accessable again.
Here is a bit of related info on a 128.