I didnt the first time either. If you talk to her and convince her to escourt you to her bedroom, then you knock her out there and walking down the stairs unavoided is quite easy.
Lady Boyle is randomised each time. I had one who wanted to go to the music room, and there were still guests everywhere.
So I just possesed her, and knocked her out in the celler, so much easier. However spring razors are fun, so I wont ever worry about that again.