Similac alimentum poop black?

We started my son on alimentum last Saturday from sensitive.. he was having bad diarrhea, mucus and he had some specks of blood one day. Well it was straight mucus day 1 of being on it.. and I thought there's been mucus throughout the week. Went to Dr with a diaper the just looked at it and they said it looks like normal baby poop and told us to stay on alimentum and sent us home. This wasn't his normal Dr, she was out this week. We'll today I changed his diaper and these black goop in it.. is this normal or should I be concerned.. the hospital and Dr seem to be brushing me off.. pictures to show what's going on..