The Litany does not counter blood magic, nor is it impossible to resist it in the first place. The problem is that you CANNOT cut a blood mage off from his resource, like you can normal mages, and as such there is no way to counter them.
And we are saying, that logically speaking, during the many millenia magical research has taken place, that SOMEONE at some time, tried the very thing you are glamouring for, and concluded that it could not be done.
Yes! And that's the problem! Someone concluded it could not be done and then the mage society collectively seems to have... given up. But just because one group hasn't found the answer doesn't mean something is impossible.
We encountered Veil Tears THREE times in total. In the basement of Kinloch Hold, in the Blackmarsh, and at Warden's Keep. ALL these locations had unique circumstances connected to them. Veil Tears ARE rare. That is the cold hard lore saying so. Just because our warden is (un)lucky enough to run into them multiple times does not prove otherwise.
Every time we encounter undead, demons or even sylvan without mages to summon them, we're encountering Veil tears. Small ones, but tears nonetheless.
Given that we encounter them a
lot, tears are common.
The Litany only counters a subset of blood magic (mind control spells) and it's only able to disrupt the casting of those spells too.
But it shows its possible to counter it. It's only the absence of further research (because paranoia) that prevents it from being developed further.
Same thing for the Veil, really.
So creating classes for mages/templars like defense against blood magic similar to the defense against the dark arts in harry potter would be impossible?
Not impossible, it just requires a major reform of the current rules regarding blood magic. The only way to learn how to counter something is by learning how it works, which means that those who want to find a counter to blood magic -even a counter that can be learned without learning blood magic like the Litany- the mages developing that counter first have to learn how blood magic works.
You can see how this might be a problem in the current system.