Air carrier Captain reported maintenance personnel attempted to load unlabeled COMAT in passenger cabin overhead bins. Unlabeled COMAT was refused.

Narrative

On several occasions Maintenance has attempted to load unlabeled COMAT (Company Material) in the passenger cabin over head bins. While the flight operations manual) and Flight Attendant manual) have no clear guidance on this; the AOM (Aircraft Operations Manual) strictly prohibits COMAT to be loaded in the passenger cabin. Also; per AOM; COMAT must be labeled and if over 200 lbs.; Planning must be notified. We have been informed by word of mouth only; that the unlabeled boxes contain plates and setups for meal service. The packaging used doesn't give any indication of the contents and are usually recycled from other shipments. The boxes are individually heavy; however no other info is provided. Maintenance loads the boxes in the overhead bins and usually it is our Inflight crew that notifies us that they are on board. Because of the undocumented nature if these boxes; I have refused to transport them each time. Also; these COMAT shipments are traveling internationally and are not included in any customs paperwork (as required by the company) and there is no proof of who loaded them; value; purpose; or their contents. Although I have only received minor push back from Maintenance; the prevailing mentality is that they will try again and hope the next crew accepts it.