FWIW, I'd call Norton's style the actual Philly Shell, as it was an adaptation of what Frazier used (also taught by Futch) who was a Philly-based fighter at the time and that's where Futch had strong influence due to Frazier's popularity. Also, passed down through Georgie Benton, who trained Bennie Briscoe, who used a similar positioning and approach to fighting as both Ali and Norton.
Now, WAS all this stuff influenced by Blackburn? Most-likely, Blackburn and Futch knew each other well as Futch was a regular sparring partner for Louis, even though he hated it only being a middleweight himself. Joe always requested Futch because he couldn't land punches on him. But therein lies the separation, it's likely Futch picked up defensive nuances from another source in Michigan, as he learned to box in Detroit (which I think is why they sort of look like practitioners of the Crab style Floyd uses). Then took them to Philly and applied them to the aggressive fighters there. And cemented the spread of that with Benton becoming a renowned trainer. If anything I'd say what looks influenced by Blackburn is how these men threw their punches. They dropped their weight similar to how Louis did when they wanted to hurt you.
IMO the most glaring weakness of this style is two-fold, but centered around one thing, which is that these men tended to drop their weight onto their front foot. But it's clear it had a function, they'd bait a punch, and crack you when you opened up. But if you knew how to stay back and fire from just out-of range, they could be out-boxed. That's how Briscoe lost almost all the fights he lost, and fighters who didn't give up control of distance to Benton (weren't baited by him) gave him the most problems. Also, for the hyper-aggressive ones like Briscoe and Frazier, they could be put off-balance with a shove or nudge in the right direction, they'd have to work to reset before starting anything else. Foreman did that to Frazier for most of the first minute of their bout. Just shoved him off-balance every time he went in, a-la a teacher of Foreman...Sandy Saddler.
They were all pretty adept at slipping, sliding, and rolling with punches, but some of them were a bit too rhythmic. They could be timed, albeit it took some of greatest fighters in the History of the Sport to time them.