We had a black snake in our nest box a few months ago. Only I was the "lucky" one to find it. LOL I screamed for hubby, who is more afraid of snakes than I am. He pulled the thing out by the tail, and it kept coming and coming! It was over 6.5 ft long! OMG, hubby was freaked. He couldn't kill it though. We put it in a bucket with a lid and relocated it to MIL's property.... she has a stream, wooded area, and no chickens.
My friends had a problem last year though... they took a huge blacksnake out of their nestboxes 3 times in 2 weeks. After it kept coming back, they ended up killing it.