Being meowmie to two blackies, I know first hand the prejudice they are subjected to. I work at a shelter and year around I show people a blackie and they say "oh not a black one". Not to be offensive here but many are Asian families so perhaps the stigma is more prevalent in those cultures.
I did have a family looking for a cat as companion for their elderly mother and said no blacks because they are hard to see and she might hurt it by accident..perfectly understandable. We don't adopt out blackies in Oct.
Anyway, I think the real danger to all cats but particularly black ones near Halloween are sick people/kids that think it would be fun to torture one as part of a Halloween "prank" or "trick". Sadly in many cases it has less to do with religion or superstition than down right cruelty In regards to adopting blackies, some people just adopt one as a Halloween "decoration" and abandon/ignore it after.