I had mine blocked up as well with the mud/substance. Not sure exactly what it is. Maybe a PO had put stop leak in the radiator or something similar. I flushed out the heater core with a garden hose both ways real good and the problem has not reoccured.
Morning, bucky. "LOOKS" like some old desolved rubber and other normal crap, the stuff that usually collects in heater coils and such. Also some leftovers from block cleanining. Did you do a block flush (everything hooked up) after your overhaul? I sometimes find loose stuff after a getting a block cleaned. Depends on how clean a shops vat is, length of time in and/or temp.