Bosch tumble dryers have black plastic bearing strips on front drum support that wear and can give off black threads. Could be this. To replace these is not a d.i.y project unfortunately. Cheers
Kenjo, August 2012
wipe between front drum and the front mounting/bearing area from inside the dryer drum.could be worn front bearings or a faulty drum.put clothes in a laundry bag to dry if problem persists
