Black cats were thought of as being related to witches, and witches were thought to take the form of black cats when they wanted to be discreet. Thus, it was bad luck to meet a black cat.
The superstition is that the black cat is a familiar of witches and can talk. It is also thought that black cats are bad luck. If you see a black cat in the path then you have to go by another route.
I'm not sure if anyone knows why they started. Each country has their own superstitions about black cats. Most think they are good luck or magical. Americans are about the only ones who think of them...
it is not superstition but warning that something is going to harm or death awaiting only during that time it cross the street or run sideways when we walk on the street.
The bottom of all this to my opinion, is the colour black. It was the colour of the old European (Roman, Celtic German goddess of fertility Mother Earth, giving the later "black madonna's". Which was...