My diarrhea is black,, got tested for c diff today,, have had diarrhea for 3 weeks,, can this mean blood in stool? i'm very tired and nauseous!! very

Tarry?: Blood in the stomach is cathartic (leads to diarrhea) but usually, turns the stool "tarry" in consistency. So, yes, a change to black liquid stool can be a sign of bleeding in the GI tract. Fatigue & nausea are also consistent. The bleeding may be sporadic (ie, slow bleed) but still cause these symptoms, so it behooves you to get checked out by a physician. A simple stool test will confirm blood.