Several days before my half stool was black and terry(not before and not after). around menstrual periods i am a bit bloated. my cbc was perfect. i read somewhere that bloating and black stool can indicate cancer. does my symptoms sound like cancer?

Non-specific: Tarry black stools are indication of bleeding in the stomach and/or intestine. Even though it happened only once, it would be prudent to have it evaluated and you may need colonoscopy or imaging study. Bloating is entirely non-specific. "Perfect CBC" does not rule out abnormal bleeding or cancer. By the way cancer is unlikely at your age. Wish you good health!