This is like saying a black cat that crosses your path is bad luck to quote Groucho Marx "A black cat crossing your path signifies that the animal is going somewhere."
In short it's a silly thing to think black magic will happen or bad luck will happen because it's Friday the 13th.
It's all just silly superstition, I mean something bad could happen, but that's like saying if something bad happens on December 25th one time that it must be a cursed day. There's just no science to back the claim up, and I also don't believe in luck as it is anyways.
There is no science to back up the idea that this day is bad luck. The reason some people are convinced it is, is because they make it happen themselves. "Bad luck" happens everyday, but because this day is popularized, when bad luck happens, it feeds into people's imaginations that this day is actually a bad luck day. If you are always looking for bad luck, trust me, you will find it.