Once there is a bleed in stomach/gut it can produce black tarry slools
for about a week but the density of black color fades towards the end of this duration.
If you had the surgery this tuesday then nothing is to be done and if you had it on tuesday 8days back then there might be a surgical site bleed either due to a slipped ligature on the artery
or a surgical site infection resulting in bleed.
In latter case you need to get hemoglobbin measurements daily, if level is stable then it's delayed clearing of old blood, if Hemoglobbin level is dropping then you may need blood transfusion
and antibiotics/re-exploration of surgical site that your surgeon will know.
I hope this advise would be helpful for you.