I did lights for a production in a 80 seat black box, I don't remember much of the set design, but I may be able to answer some questions or ask some of the people who worked with the show. I believe one of my current students worked the run crew.
The Black Knight's fake arms (one with a plastic sword attached to it) were held on by tie line which he held in each of his real hands. When each arm got cut off, he released the cord and the arm fell off. Then Arthur chased the knight behind the set, and in very dim light, a stagehand opened the castle doors and set a black velour covered flat in the door opening. The flat had a hole for the knight's face with a helmet above it, a torso, and two legs held on by magnets. A strip of metal was at the top of each leg with a matching magnet on the upstage side of the flat. The knight would pull the magnet away when each leg was chopped off and it fell to the floor. Sounds hokey but the audience really laughed. Didn't hurt that the actors did a great job timing things as well. A couple of times the timing was off and the legs fell a second or two after being severed by Arthur's sword. Made it even funnier.