It is possible that the particles can be coming from the food you are eating. It may be possible that parts of the food may pass through undigested.
You can consider a home stool test to test for blood - there are many cases of bleeding where blood cannot be seen by the naked eye.
If there continues to be concern, you can consider an endoscopy such as a flexible
sigmoidoscopy or colonoscopy.
Coffee-grounds refers to the presence of old blood or gastric juices leading to the "coffee-ground" appearance. It is a marker of upper GI bleeding and can be found in vomit (i.e. "coffee-ground emesis").
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
Medical Weblog:
kevinmd_b