Why can't I make a black soul gem in Oblivion: Elder Scrolls IV?

I've been playing the same character for a very long time. In-game it's been 3 years since I started her. (I don't know why it's taken me so long.) but now I'm trying to make a black soul gem. It's day 983. I calculated that the next Shade of Revenent should be on 985. But it's not coming. I've tried days 984, 992, and 993. The light just isn't coming. I've only tried the Dark Fissure altar, though, so maybe it's glitched. I'm not sure. Will it stop showing up after a certain amount of time because they didn't expect anyone to play so long or what?