Black specks may be seeds, foods, or from die off of yeast or bacteria.
If you start any supplement that might create looser stools, temporary diarrhea, or die-off of yeast or bacteria (like an antibiotic, probiotic, digestive enzymes, antifungal, laxative, etc), you might see dark or black flecks in stool during this 'cleaning out' period. Certain types of adverse bacteria in the colon can produce dark residues and this is getting cleaned out.