I have ulcerative colitis, i usually have light blood when i go through a flare but past 2 days i've noticed black blood or mucus on tissue too???

Not usual for U.C.: If you recently started taking Pepto-Bismol or Iron supplements, they both can turn stool black. But, if this is not the case then you should see your physician as soon as possible, because black stool(Melena) indicates bleeding from the upper gastrointestinal tract (stomach and first part of small intestine). You can have bleeding from an ulcer or gastritis without having any stomach pain.

Contact your doctor.: I would recommend that you contact your own doctor regarding this. He/she knows you and your disease best. Black blood can be clots, which can signify more bleeding than you've mentioned; mucous can be part of colits, but could also be a sign of infection.

Call GI or your GP: If you are worried about a flare with symptoms that are atypical for you, contact your treating physician. A little blood is not as worrisome as black stools (melena) and mucoid stools. Your doctor may want to run tests to check for anemia and inflammation or prescribe a medicine to help with your flare. Be sure to hydrate well and to use your usual pain control regimen.