furymom! Were you staying on the beach near Paradise By the Sea when you saw the black stuff? If so (and even if not) I think I can answer your question. That black stuff is peat from ancient cypress forests. 
The beach took a battering from Ivan in '04 and Dennis in '05, exposing a lot of peat. The layer of fine white sand is changeable -- it builds up over time -- and sometimes more of the peat shows through than other times. Hope this helps. 
Not really ... just an avid SoWaller who is really familiar with that beach!
You so smart, TFT.
. Hi guys, miss you! furymom. TFT is correct but the beaches have filled in beautifully from the storms. Sand has come back but if it was within about 6 months or so of Dennis that you were here, there were people using darker sand to renourish. So much of the dunes was lost after Dennis that we ran out of sand when people started filling it back. For a period, the county lowered the color grade of the sand (not by a whole lot) but there was a big thing on the board about it with terrible photos of some owners using very dark sand that didn't meet the color requirement. Anyway, that's gone, beaches are snowy white and beautiful again. 
