Black is a sign of tissue death (necrosis)- there was likely an injury to the tail where it begins to turn black, and the end will likely shrivel up and fall off - Can you post a pic? If its a large section, you will want to take her to the vet as a precaution.
I had a similar sort of thing on my male linneatus - it appeared to have been from a bite to the tail, manifesting itself as a slighly grey "u" shape (hence thinking it was a bite) then the tail filling with fluid, the fluid sac slowly went down after 2-3 days with a lot of showering (perhaps incidental) and shriveled up, then fell off to reveal a slightly shorter, but perfectly healed tail stub. Pics of this process below.
Notice the odd look to the tip of the tail (extensive dehydration as well)
http://www.wildeyereptiles.com/cpg132/albums/userpics/DSCN7898.JPG
Fluid sac
http://www.wildeyereptiles.com/cpg132/albums/userpics/DSCN7905.JPG
Drying process
http://www.wildeyereptiles.com/cpg132/albums/userpics/normal_DSCN0082.JPG
I dont have any new pics of his healed tail, here he is now...
http://www.wildeyereptiles.com/cpg132/albums/userpics/DSCN0852.JPG
-Leah
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Chameleons, geckos & invertebrates