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
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