Can metformin cause ulcers? I had diarrhea but there was black/dark liquid (blood? Can't tell) among normal-looking feces. I take victoza (liraglutide) as well.

Not likely. Although the most common side effect of metformin is diarrhea, it is usually not bloody or dark. Victoza (liraglutide) can cause nausea, occasional vomiting, but not ulcers or bleeding. You should see your doctor if this continues.