Blood from color (cancer) would be red
in any case. Gut
blood has black color only, if it originates from the gaster or esophagus (
in any case it has to go through the gaster, where acid changes it into black). You may want to check, if your test can detect this acidic blood. If you can't get rid of a feeling it's blood, go to some lab. It may be from polyps, or from crohn's disease or...27 y. is young age for colon CA though (except if it appeared at young ages in your family).