Brief Answer:
Further investigations needed to find the cause and management plan.

Detailed Answer:

Hi,

Yes, the black color of stools could be due to skins of black grapes....still if recurred without eating black grapes, blood in stools needs to be excluded while stool examination.

Alternate diarrhea & constipation after 1 year of surgery for diverticulitis can be-
-Some intestinal infection (especially bacterial or amoebic).
-Diarrhea/constipation can be due to recurrent diverticulitis (postoperative incidence being 10%).

-You need investigations to reach to the certain cause, i.e.CBC, stool examination, abdominal CAT-scan & get specific treatment... (with suitable antibiotic/anti-amoebic drugs)
-Self-care with having a high fiber diet and sufficient water intake (avoid laxatives) along with weight reduction(if overweight) would help.

In this context, I'd suggest you consult your gastroenterologist and follow the treatment plan offered along with self-care.

I hope that answers your query. Any follow up query is welcome.
Thanks.

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Brief Answer:
Cause explained down.

Detailed Answer:

Hi,

Black grapes contain fibers in addition to vitamin C and minerals. The fibers (in pulp and skin) retain water but do not to get absorbed and cause clearance of bowels.
Skin of the black grape does get finely pasted but not digested that is why it can colour stools.
It seems You were going to have constipation, on the day reffered in the query but the fibers in grape caused mass peristaltic (movement of large intestine to cause bowel movement). This caused a comparatively looser stool and this was mixed with mucus in stools to make it stringy and sticky so could not come out easily and you had to pull it out.

I hope that answers your query. Any follow up query is welcome.
Thanks.

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