34/f hist of pcos. taking metformin. tracking cycles for 3 months. positive opk cd21 w/dark (almost black) spotting. is this a true positive result?

Sounds like it: If you do a home ovulation kit and it is positive there is no good reason to think that it is a false positive. You could have a Progesterone blood test done by your doctor 7 days after the positive result. If the level is above 10 then it is a true test indicating you did ovulate. You could also check a pregnancy test in 10-14 days. Good luck.