If black walls are your pet peeve, then I totally agree with your that Knott's and even Fright Fest are far superior. But their downfall also comes from the fact that so many of their mazes are exact or nearly exact for years. They're enjoyable, but there's little repeat visit factor (although Fright Fest does still get me to go just because it's so incredibly cheap by comparison--but seriously, they're the worst at it. Willoughby's has been exactly the same since it debuted in 2012). I tend to only visit Knott's every other year, because going yearly just means too much of it being the same as last year. At least with alternating years, about half the mazes are new.
So it's a double edged sword... HHN does have a lot of black walls, but they also have a totally new experience every year. Not defending it, but just saying there's a price to be paid for having no black walls--and that's having the exact same maze for years. Based on how the HHN crowd has reacted when a maze has repeated, I don't see it going over well (although we'll have to see if that reaction is different if Universal Monsters does end up returning).