I am currently using moth balls around my coop, as I have 2 week old chicks out there with their momma hen.(they are surrounded by harware cloth, but I don't want a rat snake slithering around trying to get in.
The mothballs have been out there about a month, from when she was setting on the eggs, now chicks are almost 2 weeks old. Havent seen a snake! Does that mean that mothballs work? well, of course, no!
In my online research, there was some gov study that showed mothballs don't actually work on snakes; however there is just so much anectodal about it working. I've read over and over about ppl who put out mothballs....never saw another snake!
I put them around where chickens can't get to them, yes, they are toxic, no pets/kids should be able to access them.
also, the black you describe sounds like a speckled king snake,maybe?
its a shame you killed the snakes, they are benficial animals and not venemous. But I know I'd be mad if I saw one with one of my chicks in its body! Still, my son is a snake lover and we just wouldn't kill it around here.
But, if an animal is not endangered/threatened then by all means dispatch it, of course. I am not that ridiculous about these things. You did what you wanted to do with a predator. There are plenty of Texas Rat snakes and King snakes out there slithering around!