What Causes Black Stool When Suffering From Diabetes?

My spouse is a 66 years old, diabetic. At about 7:a.m. his stool was black. The oncology nurse advised me to bring him in if it remained black. I have been watching his blood presure and it was 112/64 , pulse 85 at about 7:30 p.m. Over the course of the evening he laid on the couch watching TV. His pressure dropped and averaged 92/53, pulse 85 (checked hourly). He has said he was feeling fine. He is asleep now and his pressure is 86/51, pulse 82. Is this too low and should I get medica attention now or do you think he will be okay until morning when he wakes up.