Today 1/2 my poo was black (hard smooth, pebble-like, not a tarry texture) and 1/2 brown (softer). i broke the black poo open: it was black only outside, while dark brown inside. if there were digested blood, could it be black only on the outside?

Maybe!: Blood in the stool can be in large or small amounts. Old blood (from higher up in the GI tract) is often dark maroon, nearly black, or there can be bright red blood in the stool, in large or small amounts. The only way with dark stool to be sure is to collect a sample for testing to detect blood. What you describe though, may not be blood at all, but due to something you ate. See MD.