Hello and welcome to healthcare forum.
As you said that you had
flu recently, for which you might have taken
analgesic medications.
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.