To start this off, I don't believe in either, but if I believed in pure luck then I would say they are good luck. Twice in this year, 2015, a black cat crossed my path and I experienced normal, maybe even better than normal, days on those times. I am for the black cats!

Superstition often developed out of false beliefs and the belief of witches transforming themselves into black cats in order to prowl streets unobserved became a central belief in America during the Salem witch hunts. However there are some cultures where black cats are considered to be good luck

Walking under a ladder was considered to be bad luck - that was in the days before workplace safety where it was a dangerous place to be standing.

I love cats. I also have the sweetest black cat ever. If I was in an alleyway and saw a cat, I proably would run towards it and try to hug it.

But seriously, luck can be broken down into pretty much just random events occurring in a secession. There are 2 non-dependent variables in this case; the black cat walking perpendicular to you and an accident or tragic event occurring shortly afterwards, it has nothing to do with the cat or even the coat color as the variables are non-dependent (assuming, of course, that the cat doesn't directly hurt you)

But also this is kinda racist to the black cats. Kinda history repeating itself but with a new species...