I've only created 5 black soul gems through the altars. However, I think I have about 30 now, and I haven't made any more. When I kill vampires or Xivilais or other creatures that qualifies as grand souls with Umbra, I get a new filled black soul gem from out of nowhere. Is this supposed to happen, or is it a bug? It doesn't happen every time, but most of the times.
Also I think this happens with greater soul gems as well.
On a similar note, the enchanted chest does not produce ingredients anymore. It did the first time, but not anymore. I have a flax seed in it, btw. Not sure if this is relevant, but it can somehow seem like instead of giving me extra ingredients, the game gives me extra soul gems...
Do not take me for a conjurer of cheap tricks!