Black Zetsu soloing Madara wasn't a plothole. If he had killed Madara in any other way, THEN it would have been a plothole. Black Zetsu used Madara's one pre-established weakness against him: his blind spot. What it was however, was just bad writing on Kishi's end. Kishi had made Madara way to powerful; so powerful that he couldn't write his way out of it, so he just killed Madara through Black Zetsu -- which ironically, only caused Kishi to write himself into another wall, as it set up Kaguya being even more powerful than Madara. Really this whole issue wasn't created when Black Zetsu soloed Madara, but when Madara's character was introduced in the very first place. Kishi has been hyping up Madara so much, that when the time came to bringing out Madara he had only two choices: 1) Have Madara completely fail to live up to the hype, 2) Have Madara live up to the hype and solo the entire Allied Army. Kishi tried to do both choices by having Madara live up to the hype only when it was convenient to the plot. Otherwise Madara just stood off to the sidelines and did things that were completely illogical to his character and motives.
