"Besides the fact that some people think they are sacred, why does black coloring make them so expensive? It always seemed odd to me."
Its like anything else in life. Everyone puts their own value on something and at the end of the day its a supply and demand thing. IF someone wants an all black chicken and they like the looks of them and the idea of owning one then they are going to buy one. If they then go looking to buy one and realize they are somewhat scarce then they have a decision to make, either pay the asking price or not own one. They are free to choose whatever makes the most sense to them.
Another thing that does make them somewhat more scarce is the hens dont lay as many eggs as "typical" hens would.
But you can make the same argument about people who rush out to get an Iphone 7 when their Iphone 6 or even Iphone 5 works perfectly fine. Or why someone would rather buy a Lexus than a Toyota, or Acura instead of a Honda. You can make arguments that functionality is more important than the things that make something new and exciting.