The thing about black cats – as with the thing with mirrors (broken or otherwise), the thing with ladders (and going beneath them) and the thing with salt (generally, but not always, spilled) is that black cats are, by their nature – by several of their natures – accretors of energy, at least, of certain kinds of energy.
Black cats, you see, are children of the Void in a way no other cats (there is some argument here; some people would like to point out that specific other cats are also children of the void, especially specific Persians). They carry within them a spark of magic, a spark of belief, a spark of the mystic.
Not specific black cats, not familiars, not the black cats who live in bookshops – every cat born black as midnight, black as coffee, black as a moonless night in a coal chute. Every black cat is born with the spark of magic in them, more so than any other cat, because they have been touched by lady Nyx herself. They have been touched by the core of the walnut tree, the blackest ink. They have been touched by the depths of the sea. All of this, every black cat knows, somewhere deep, deep in her little fuzzy soul. (And you can tell this, looking at them, if you look exactly at the right moment.)
So what does this mean?
This means that you never treat a black cat with disrespect, of course, because it will bounce off of them like so many laser dots off a mirror and it will stick to you like you are glue.
But it means more that that.
It means that if you spend time around a cat, especially a black cat, all of your actions, all of the, ah, vibes, that you send out into the world, they start to accrete. The cat acts as a holder for all of that, while not taking any of it into herself (or much; a black cat will become as nasty as any other angry cat if she spends too much time around the wrong sort of vibes). They accrete, and then, like cat hair, those actions shed off onto you.
So a black cat is not inherently bad luck, no.
A ladder one walks under is not unlucky, either. A mirror does not hold your soul, and salt – well, salt is best left for its own story some other time.
But a child of the Void will always reflect one’s actions back on one. Of course, the world itself will do that, in time, it’s just that the process is faster with cats (as most things are).
For a kind-souled person who is giving, a black cat is a loving purr machine, a warm look at the night-time, a pool of darkest hot cocoa puddled on the foot of the bed.
I think you can guess what a black cat can be, when she crosses the path of that other sort of person.