One can find, easily, a LOT of NOS original, new still in the package, Michigan license plates. They don't cost too much, and you can legally use them on a vintage car provided the number isn't already in use. I think the state offloads unused plates, people buy them and hold them, and then make a business years later reselling them. Gee, what a concept; like a generation skipping trust!
The relevance of a black plate car in California is a bit different -- it shows that the car was a California car prior to 1969/1970. Yeah, it's cool because it's an old license plate but it's more cool because a 1960s car in much of California is less likely to be a rust bucket than a 1960s car in Michigan. So people from around the world would rather buy a black-plate California car than a blue plate or white plate California car. Actually, that's not accurate. People would also rather buy a blue plate car of the appropriate years for the same reason.
Opponents in California to the new issue black plates are upset because now it's easy to "fake" having a California-original car.
There are other ways to get black plates on a CA car that look real (because they are) but it's more complicated than just ordering the reflective plates from DMV.
The interesting outcome of this is that since the Blue plate required registrants didn't reach the number needed, only "true" blue plate cars (1970 - 1986?) will have blue plates. So they can't be faked. They also can't be faked using the other DMV methods (assigning a pair of old plates to a car -- that ends at 1969).