I can appreciate a account wide block for the sake of saving spoons, so to speak. But I think the way tumblr does it is pretty decent. You have a list of blocked accounts in the setting, but if you want accounts blocked on side blogs you need to manually add to that side blog's block list.
Being able to add the url list of blocked blogs would be nice. As oppose to not having to refresh the page, see potentially triggering content, and hit block for each side cohost blog. As I had to do this with someone posting about a rape fetish with some graphic content at the top of the page.
Coming back to say actually I want a single account wide block list to input a url once and be done with it.
I don't want to constantly see a literal transphbic slur as their url every time I have to block someone on a different account. Like I just had to, right now.
I definitely think a checkbox or something to block someone on all your accounts would be ideal. There are circumstances where I'd want to do so, and others where I wouldn't, so the ability to easily choose between "block from this page" and "block from all my pages" (or whatever) would be very nice.
"maybe implementing some sort of "superblock" that only lists the name of the main page of the user? "
I know of one website that does this, that I won't name*. It is possible to block a side blog and have all the blogs associated with that account anonymously blocked.
Now I do understand the concerns with implementing such a thing. Like, it'd be very easy to stagnate the whole site by too many people blocking each other. But I think there's enough people on cohost that it wouldn't be a problem. Or at least, maybe this Superblock could be artificially limited? I don't know. It's a thought.
Also in this circumstance, it might be worthwhile just so staff doesn't have to get involved every time someone evades a block with a new sideblog, like in your situation. Having to mitigate 'they said / they said' between users is always a complicated situation.
*it was run by a scammer who stole 4 usd figures worth of money, allegedly.
My main concern would be that a superblock would be an easy way to test whether two pages belonged to the same user. If you suspect that a page is owned by a certain user, you can block one and check to see if the other is blocked too. I guess you could make it so you just couldn't see whether a given page was blocked via another page's superblock, but that seems like a mess.
Personally, keeping page ownership private would be the highest priority, followed by being able to tell which pages you have blocked, followed by being able to block all of a user's pages. If it's possible to create a superblock without breaking the other two priorities, then I'd support it, but I can't think of how that would work.