I could be wrong, but I can't imagine Blunt damage is very effective against armor such as leather + chainmail, however I'll bet it is extremely effective against armor like lamellar and platemail.
For starters it's important to acknowledge that
all good armor used layers of padding beneath that would greatly help absorb a blow. This went for both mail and plate.
Secondly, mail, being non-rigid, would do very little to absorb the force of a blow. Mail itself provided decent protection against piercing, and excellent protection against cutting, but poor protection against blunt impact. BUT the padding beneath it provided good protection against a blunt hit.
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Coat-of-plates, being made up of larger linked rigid sections, would provide better surface protection than mail, and if you had the same padding beneath, you would get the same protection as mail but better. Coat-of-plates is the closest thing to plate we have in Bannerlord.
Full plate (not really a thing in Bannerlord with the exception of helmets), being rigid, and with the same padding, would provide the best protection out of all of them. Because when someone hits you with a blunt object, some of the force is dissipated across the plate and the energy goes into dinting the plate, rather than being pushed directly into your chest or bones. Of course, some force will get through. But not all of it.
You couldn't cut through full plate, and could only pierce it with difficulty. So blunt damage was the
easiest way of dealing with a suit of full plate. But that doesn't mean it was
super effective like a Pokemon Fire type attacking a Grass type, either- padded plate was still great defense even against blunt force. And through a process known as "Flanderization", the best way of dealing with plate - but still not highly effective - has now become known as some sort of hard counter in video games and movies.
For a TL;DR, here is how I would say that armor works comparatively against types of damage. Each star = more protection.
Cloth: Cut -

Pierce -

Blunt -
Aketon: Cut -


Pierce -


Blunt -

Aketon+Leather: Cut -



Pierce -


Blunt -


Aketon+Mail: Cut -





Pierce -



Blunt -


Aketon+Double-Linked Mail: Cut -





Pierce -




Blunt -


Aketon+Coat-of-Plates: Cut -





Pierce -




Blunt -



