Feather dusters with black tubes...

So I thought it was detritus at first but tonight per geoff's description in another thread, I noticed all my feather dusters tubes are black tonight. They seem brittle but I haven't checked yet. I have piles upon piles of the tubes that have shed off (I siphon them out every other day). A few days ago they were white, the only thing I have done since then is siphon up the detritus and it's a 10g bb as well. Tests tonight showed 8.4ph and 30 ppm nitrates (Thursday they were 10). What causes a black tube for a feather duster? What are the tubes made of? I searched the net for a good 3 hours for a situation like mine and found nothing close. There are about 7 dusters and have all shown movement growth and normal reactions. It seems the worms theirselves are health but just not the tubes..