@feruze-- Yea, people who have never seen coffee ground emesis before can mistake it to be something else. (Coffee ground emesis is another name for this condition). I've mistaken chocolate remnants to be blood before and the other way around too.
I have two ulcers in my stomach and I've vomited coffee ground like blood a couple of times. My doctor described it to me in detail so that I would always know when it happens and can get treatment before it's too late.
He said that when my stomach bleeds, the blood gets digested by my stomach juices. The iron in the blood is oxidized in this process and turns the blood into a dark substance, what looks like coffee grounds or spots of a blackish-brown dark substances to us.
When I'm in doubt, I still go get checked out. I've been told that if it's not treated in time, my stomach acids could leak out of the ulcers and eat away at the surrounding organs. Doesn't that sound horrible? I always get scared out of my mind when I vomit coffee grounds.