What does black stool mean?

Hello everyone,
starting from about 4 A.M., I woke up in the middle of the night and had stomach pain. When I woke up, my stomach pain became worse and worse. I also had a headache. Afterwards, I finally had the urge to go to the bathroom to poop. However, I had a surprise – I saw blackish and dark greenish diarrhea.
I don’t know how I got it. I went to a trip and just returned the day before yesterday, is it a stomach flu? Is it something very serious?
Update: After going to the bathroom and having diarrhea, my stomach feels a lot better. It doesn’t hurt anymore.