Welcome to the gastroenterology community! I would recommend seeing a gastroenterologist about this. Black stools can indicate bleeding in the upper part of the digestive system. You should probably have a colonoscopy and endoscopy to investigate this. Good luck.
do you think it could be serious? With the blood thing before and now this what do you think.
It could be serious or it could be something not too bad. The only way to know is to see a doctor and get some testing done.