Yep. Although teh chap in the Golden Cat mentions Lady Boyle always wearing black as well (only noticed it when playing that level again....) Getting the Dunwall tower skeleton key there made the next mission much easier as well.
Did you locate your target by talking to a certain lady and getting her a drink?
That costume party level was brilliant - first one I tried to do 'properly', without being seen.
I went for a dander upstairs but didn't find much, if it wasn't for that person mentioned above I would have just took them all out