If all porn is like this and all porn watchers are watching it, it ceases to be worth much blackmail value so in this case I don't think so. That's why they want real ID laws attached to porn viewing so they can find the actually abnormal or non-average stuff people seek out and try and use it and people's shame about it to blackmail them.

No one would give a shit about someone being revealed to have watched (fake) incest porn 5 years from now and even trying to slander them as watching incest porn would have a good amount of people questioning whether it wasn't just the fake kind. It's the fact your hypothetical blackmailee has a foot fetish and an interest in heavy bondage that will be used to paint them as a wacko. Not this. Many people have a non-mainstream interest. Failing that, eventually they will have clicked on something they don't even enjoy just by chance and that can be used to insinuate things about them.