Kevin,
Fresh powder is a very light gray. I think it is moisture and oxygen that will eventually cause it to turn a dark gray, almost black, and clumpy. Here's a thread where I looked into this a while back.

http://www.poolforum.com/pf2/showthread.php?t=4659

I don't have the info on the replacement at my fingertips but there is info on all that in the forum dealing with the kits.

Al