Tires shedding black threads
The word is SPRUE and can present as a short rubber "hair" 1-2 milimetres thick, or a long thin straggy line around the tyre, somewhat like a fish fin.   

If your rim tyre is too narrow for your tyre then its possible for the edge of the rim to wear on the tyre.  This could produce threads that really are thread, instead of rubber.  I've never seen this on road tyres, but its a rare possibility on oversized MTB tyres on hybrid/cruiser rims.