Oblivion Black soul gems question?

I'm trying to do one quest for a daedric shrine and it requires a sacrifice of black soul gems. I've read that you make them by putting Grand gems into an altar (I've used the one at the dark fissure) and casting soul trap on them OR by waiting a week. I've tried both and nothing has happened to the gems. (And it has been done on whatever its called when the purple light shines down) Why isn't it working? Other info: The gems are stolen if it makes a difference and I've completed all quest lines but the main and Knights of Nine.