Depending upon the country, all of those explanations hold true.
Also, in some countries, it isn't BLACK cats that are unlucky but, WHITE ones.
It is said that all black cats have a few white hairs. This is because so many purely black cats were killed, during the Dark Ages. This, significantly depleted the black cat gene pool.
Of course, the mass cat exterminations also, led to the depletion of the human gene pool. Fewer cats led to a rodent population explosion, causing the plague epidemics, of the times.