I agree with Meowzer, it's totally normal. I made the mistake last time of just plopping the new bag of Chemi-Pure in the sump and a black cloud came out of the output nozzel for a couple of minutes, and then it cleared up. Luckily it didn't hurt anything. I was soooo scared it would though!
When I do a weekly water change I always make sure to lightly rinse the bag of Chemi-Pure, black stuff still comes out even then.