I used chemi pure for a long time with my first tank. I never ever had any "black soot" to come off of it. Did the bag possibly break, or tear? I rinsed mine really well under just plain tap water (at the time that is all I had) I stoped using it and now I am a big fan of purigen by seachem. It seems to work alot better, last longer, and is regenarble. Much more cost effective as well. I would try to remove as much dust as possible if it were my tank. I dont know that its bad but I cant think of a way that it would actually be good for things so I would try to siphon it out if it were mine. HTH Steve