There is a discrepancy between the Litany used in Origins and the one used in Asunder. In Origins we are told that it must be recited in order to prevent possession through mind control. In Asunder it is claimed that it was invented to dispel demonic influence and can be effective even after it has taken place. In Origins the mind control was being practised by a demon. So it would appear that the Litany is meant to be effective against demonic control of the mind. Therefore, unless you are willing to concede that all blood magic control of the mind is done with the aid of demons, the Litany will not be effective against all types of mind control through blood magic.
I would agree with the poster, though, that it seems odd that the Litany is not more widely available/known about. You'd think it would be something all templars would carry with them. Also, if mages were taught it, then they could recite it whenever they were in a situation they felt at risk of possession. Instead it would appear that it is something the Seekers keep to themselves, since it is known that they have some form of protection against mind control and the Litany would appear to be it.
I'm inclined to think this is probably just another instance of inconsistency in storytelling.