Hello and welcome,
Though it is difficult to clearly distinguish sometimes, but black food usually do have some similarity to the eaten stuff and would be in particles. While the black flecks are thin net and usually float.
I would like to put some points for the stool caliber you have been worried about;
1) Is the caliber of the stool very important and diagnostic of any condition; NO.
2) Does the caliber alone can conclude for any condition, specifically cancer; NO.
3) Would a cancer which has changed the caliber of stool (the growth inside forms a typical shape of the stool), will have variable caliber of the motion; NO.
4) So, what is the explanation of the caliber of your stools; may be there is not much reason and it is just dependent on many factors, like your meal constituents, your water and other drink intake, your physical activity and your mental state.
Am I concerned? No.
Would I hurry to get some investigative work for this in you or any of my patient; NO.