Sounds like the bolts were loose..A 19 pitch is way too much for a 44 twin..No wonder the top rpm is 5500..We use a prop like that on our 80cc racers..If a spark plug blew out it had to be loose, too...Was the engine new ? Maybe you should send it back to RCS.......
Black "stuff" sounds like aluminum wearing on aluminum, maybe it scuffed a piston when the prop bolts sheared.....