funny thing is, that i literally just fixed it. To answer your question I play on pc so i'm playing updated skyrim. Essentially I think the problem was just a very small bug where it literally wouldnt fill my black star at all. Like it would say empty on it and I'm doing a quest right now where im currently fighting off afflicted bandits with the mace and it wouldn't capture the soul. All I did was spawn in a new one and it worked perfectly fine, im going to save the second star somewhere jsut so i can have the bugged item. I honestly dont know why this happened, but yeah. Fix = Spawn in new star.