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.
do you think it could be serious? With the blood thing before and now this what do you think.
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.