I have tested this twice, both times I had no empty grand soul gems and I had an empty black star. I soul trapped a mammoth and killed it and my black star got filled with a grand soul. I thought the black star was only for black souls!
I know mammoths do not have black souls as their souls normally fill into empty grand soul gems. Think this is some sort of a glitch or bug, has anyone else experienced this?
Best Answer
According to the UESP wiki, the Black Star should only get filled with black (i.e. humanoid) souls, but due to a bug/glitch (as you said) it also accepts 'white' (i.e. non-humanoid) souls.