Why are my faeces black in color after eating Oreos
This is a perfectly reasonable question based on a self-adminstered (human) trial. Morever, the proposer bases their observations not on one subject but several reports. The impromptu survey of friends etc. is also acceptable. By the way, self-reported intakes/output records are an accepted method in nutrition research.

My 2-year-old child exhibited the symptoms described above, after consuming two Oreo biscuits in the afternoon. That evenings soiled nappy (diaper) revealed a dark but otherwise well-textured stool. When my concerned partner brought this to my attention, it immediately reminded me of the use of carmine red and related food dyes for the measurement of gastrointestinal transit times. The principle here is that the dyes are not absorbed in the gut, so the time from consumption to excretion gives a measure of bowel function. Short transit time means no constipation and is considered a sign of great bowel function.

Based on the observations reported for Oreos, my educated guess would be that Oreo products contain miniscule amounts of non-absorbed colorants - something that is generally regarded as safe. Self-reported symptoms are generally considered valid by health professionals. I am still intrigued by the Oreo colouring puzzle, as this colourant must be natural.