Fasting: I find it easier to "not eat" than to "eat less"

I don't think intermittent fasting will make your poo black.  Black sticky poo can be from bleeding of the stomach or intestine.  Black poo with a more normal texture can be from iron supplements, or other things you ate.

It's pretty easy to check for the presence of heme in stool - I think any primary care doctor can do it.