Cool! At this point, I would say that what you are seeing is indeed old blood and mucus. I freaked me out the first time because it looked like solid stool. After the initial time, whenever I did see blood, it was in small amounts and was easily identifiable. My surgeon said this is to be expected as everything is beginning to heal because the small intestine (which is now connected to your anus) bleeds easily and everything is raw this soon after surgery. If you continue to see dark stuff or large quantities of blood, definitely call your doctor.
Are you home already or still in the hospital?