Noticed a black/darkbrown poop, mixed with light brown 50/50, not tarry. not taking meds or supplements. is this cause for alarm?

Not certain: sometimes black stool is a sign of internal bleeding. It is difficult to tell whether this is the case just by your description. The stool should be evaluated to see if there is blood present. You need to be seen ASAP for this evaluation. Good luck

I doubt it: The most normal colored stool is medium/light brown. The fact that you know the dangers of tarry stools, suggesting upper GI bleeding, is good. I suspect the dark brown may be dietary related....sometimes stool can be darker with heavy meat consumption. You say no meds, including no bismuth containing (Pepto-Bismal) substances. Safe to watch and wait--should "normalize."