It is possible that a black snake killed your chicken, but??? I don't think a chicken would allow a snake to wrap it's self around it's neck with out putting up a fuss and getting away. But I do know that my brother in law and his wife came home last evening and found a six foot black snake in the cage of baby chickens ( 4) and three were missing the snake could not get out of the cage because of the chicken wire, snake was to fat to go through the holes, well we killed the snake and took a knife and slit it open and in side were the three missing chicks!!, so reguardless what people say, YES it is possible for black snakes or any other snake for the matter except for garden snakes to eat baby chicks. How to keep snakes from eating baby chicks is a good question ( which came first, the chicken or the egg?)
Black snakes can crawl up walls, down walls or even in trees to get birds eggs, so I guess one will have to keep a good watch over their hen house!! Good Luck Roostertwo