i couldn't get through that, i had to skip it, but i didn't get to the room where the knife is
im sure all you have to do is to go thru the same room again to get the knife? when the scene ends youll need to go inside the painting lady room again and get the black object
OR youre talking about the KNIFE itself not the black object, to get the knife you have to interact with the black object and ask sai to cut himself to get the knife