most likely it is do to the special diet he is on (isn't he?) and the surgery, like someone else said, could be digested blood or intestinal slughing, I think?
basically, as far as blood from the rectum goes
bloody or normal blood red color =trauma close to the rectum or near the end of the intestines
pinkish color= usually trauma farther up
black or dark red = digested blood
thats how I understand it anyway.
If the Doc said its ok then it should be fine
I know its hard with a dog that seems to have everything go wrong with him (Oscar is like this, in his lifetime, 2 abcesses, 1 bout with severe dehydration without known cause, and chronic skin infections since his first birthday) not to suspect everything different
I have a lot of trouble myself, my bf thinks I'm "a hypochodriac" about him, and I've had one staff member at work suggest the same.
hang in there, if he's acting himself, eating and drinking, he's in no immediate danger.