As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE FLIGHT, FLIGHT ATTENDANT 1 REPORTED BLACK DUST ON THE NR 1B OVERHEAD BIN. THIS DUST LATER BECAME APPARENT ON THE COCKPIT WINDSHIELD. THE CABIN & COCKPIT HAD A CARBON LIKE SMELL. AFTER LANDING, THIS DUST WAS NOTICEABLE ON NEARLY EVERY OVERHEAD BIN. R & R BOTH RECIRCULATION FANS FILTER CARTRIDGES IAW MM CH 21-22-12. GND RAN ACFT NO BLACK DUST NOTED.