What did Mrs. Umney tell the Otis family about the bloodstain on the floor?

Mrs. Umney, the housekeeper at Canterville Chase, explains the origin of the bloodstain that Mrs. Otis spies on the floor of the library near the fireplace. Mrs. Umney says that the blood stain resulted from the blood of Lady Eleanore de Canterville, who was killed by her husband, Sir Simon de Canterville, in 1575. According to Mrs. Umney, Sir Simon survived for another nine years and then disappeared in odd circumstances, and his body was never found. She says that his spirit still haunts the house and that the blood stain cannot be removed from the floor. Washington Otis claims that this is nonsense, and he proceeds to remove the blood stain with Pinkerton's Champion Stain Remover and Paragon Detergent. As he is finishing removing the stain, there is a flash of lightning and a clap of thunder, and Mrs. Umney faints as a result. 

Mrs. Umney tells the Otis family how the bloodstain got there, how long it has been there, and how the bloodstain can't be removed.  
Mrs. Umney tells the Otis family that the bloodstain has been in that very same spot ever since Sir Simon murdered his wife at that location in 1575.  That means the bloodstain has been on the floor for more than 300 years.  Mrs. Umney explains that the bloodstain has become a bit famous and people come to visit the Canterville Chase just to see the bloodstain.  She also explains that the stain is impossible to remove.  

"The bloodstain has been much admired by tourists and others, and cannot be removed."

Washington Otis thinks that last statement is entirely false.  He claims that his special cleaner, Pinkerton's Champion Stain Remover and Paragon Detergent, will be able to clean the stain no problem.  He immediately sets to work on the stain, and he is able to get rid of the stain completely.  

"That is all nonsense," cried Washington Otis; "Pinkerton's Champion Stain Remover and Paragon Detergent will clean it up in no time," and before the terrified housekeeper could interfere, he had fallen upon his knees, and was rapidly scouring the floor with a small stick of what looked like a black cosmetic. In a few moments no trace of the bloodstain could be seen.

Unfortunately the stain returns by the next morning.