This would make an interesting adventure for players! But maybe you are asking because it all happen in the background, and your own players are in another covenant and you want to describe to them what happened to Blackthorn after the invasion ?
In any case, I assume Blackthorn would like to maintain status quo (no change in mundain relations). This will need time and ressouces, though, even for an autumn covenant.
But whatever the means, I think an automn covenant should be able to acheive this, otherwise it may be their covenant's winter.
If you want to play it out with your players, you could pick any suggestions in this topic, things your players like. I personnaly like Vetrenius' idea the most.
My idea would be along the line of Blackthorn knowing well in advance of the invasion (having spies following key personnalities): there is no "surprise" invasion of a large land. An autumn covenant, if having enough interest in mundain affairs (as is the case now: you said they posses land and ressouces) ought to have already diplomat, spies and the like in surrounding lords courts.
When a lord invade another's domain, they are in search of ally or at least neutral people from that land. I don't see why mundain nobles would not want to negociate with the nice people Blachthorn already sent to them! Otherwise, it means Blachthorn really want to battle that mundain invading army (!?)
If they can't negociate with the Normans to spare most of their belongings (lands, ressources, etc), I would suspect some other influence that is trying to make sure Blackthorn suffer from such an invasion.
If their is some other "influence group", ennemy of Blackthorn, this is where the real battle occur, and it won't be infringing on hermetic laws, because this other influence will probably be supernatural in nature (depending on your saga) or anyways comes from people actively trying to hurt their covenant (and the code allows for covenants to defend themself).