Blackish color could also be internal bleeding if I remember correctly. My cat Groot who died earlier this year of cancer had dark poop. The poops should just be brown in most cases for cats.
I don't want to be heartless to the kittens with a just get rid of them attitude, but there should be some security in being there first, it seems only fair to put the effort into rehoming the new ones.
I do see the other replies and your stepdad does seem like a dick, I'd go to war with the guy but not everyone is like me, I would totally reply to his comment about pissing all over the house with a "I'll keep that in mind when you are much older".