Basically this scene is when the new wife of Bluebeard disobeys him by opening the only door he forbade her to touch. She received a set of keys so she had access to the castle when her husband would be gone. However, when she opens that door, she finds all his past murdered wifes in their now clotted blood and the key stains in red. No matter now hard she tries to scrub it off, it magically remains there. She'll be in trouble when her husband gets back home.
Have no idea.