1. Since there is a history of eating spinach thus apart from taking iron / bismuth supplements, taking spinach, licorice or kale will easily give your stool slightest blackish color, [ black specks in stool are commonly a result of your diet]. Thus, in the best interest, it is wise to consult a pathologist or general physician who will initially go for stool examination.
2. If stool examination is normal, there is no evidence of any
parasitic infection, then it could probably be due to ingestion of spinach. However, if it is black stool, it could be due to some chemical reaction, it could be due to
internal bleeding [stomach, upper
small intestine, tumor or ulcer]. You will be referred you to a gastroenterologist for further investigations and management.
PS. Since there is a history of stool floating in water, it could be high-fat content in the stool, thus, one has to exclude
malabsorption,
celiac disease, or chronic pancreatitis also.
Take care. Hope I have answered your question. Let me know if I can assist you further.
Regards,
Dr. Munish Sood, Ayurveda Specialist