My stool contains black large flakes, say 1/4 inch in size. 1 1/2 days previous history of colon canc 15 yrs ago. Anything to be concerned?

Black stool. Black stool or tarry black stool is a sign of upper GI tract bleeding, not from colon, but anyway needs to be evaluated.
Stools. Unusual if a foodstuff cannot be blamed then GI eval in order. Unusual presentation but digested blood black. Given history concerned for later.