It is easier to add an author filter than a whole new format field that needs to be classified for every series.
Also, how would you even classify it? I consider most single digit page the same level, others do not. For that matter, not all series with similar page counts are equal, I don't think people would put the generic couple of pages oneshot on the same level as that green tinted 4 page series the name of which escapes my memory.
I mean, you just classified a collection of entries that Luoir seemed to agree with. I don't know what this "green tinted 4 page series" is you mentioned, but the question is: if people would use the new filter, would most people agree that it would cover most of the entries they intended to filter in/out? Regardless of filter, you're always going to get gray areas. Similarly, an author could also branch out from the the genre/format you intended to exclude.
And yes, it's considerably easier to add a new category to filter on, than to add an author blacklist feature for every single account to add entries to. It certainly is from a dev's perspective and if the category is applied properly, it will serve everyone (both with and without an account). And like I said before, the feature would be pretty disappointing if a lot of people will end up with (almost) identical blacklists.
But maybe I don't understand your use case properly. That's why I asked the question in the first place.
I consider most single digit page the same level, others do not.
Maybe you can expand on that a little more. What makes one entry with single page chapters different from another? Why would one person want to exclude one, but not the other? Could you point to actual entries to illustrate your point?
Maybe I'm asking really basic stuff that everyone here already understands, but I would appreciate it if you could help me understand as well.