Black oil spot in toilet when having bowel movement. on weight watchers eating fish three times a week?

When you say: "black oil spot" the concerning thing it brings to mind is melena: black "tarry" feces associated with upper gastrointestinal bleeding. This is such a concerning sign, that even if it is something else, which it might be, you have to be checked out for upper GI bleeding. Weight Watchers is a good idea; I don't think it has anything to do with eating fish. See you doctor ASAP for eval. Good wishes.