Hello and welcome to healthcare forum.
As you said that you had flu
recently, for which you might have taken analgesic
There medications are known to cause gastritis
and sometime also bleeding
from the gastric mucosa.
This blood due to its reaction with acid gives black color to stool.
You will require a course of proton pump inhibitor like pantoprazole to your gastric ulcer.
Now onwards make habit of taking pain killer alonge with antacid.
Best of luck.