perhaps it is also partly if they are kept in a group just with other black rocks?
I don't know anyone who keeps black rocks as part of a mixed flock, but the ones I know have very definite pecking order, bully the lowest hen, will not tolerate being handled by owners, and like to range!
One of my friends got 3 as 9 week olds so that she could spend time taming them, she certainly put the time and effort in but it hasn't made any difference at all.
Another group of 3 spent so much time bullying the lowest in pecking order that my friends were really worried they were going to kill her! With intervention it has stopped but the bullied one still looks terrible, bald patches and very thin, though she is bright and perky and still laying an egg a day!
Anyway, I would never class them as friendly or easy-going, not like Orpingtons or Sussex, or I believe, the warren type hybrids.