I like this idea, although the line between what armors convert to blunt damage and which ones don't might become arbitrary. Even then, it seems to be an improvement as it distinguishes solid metal plate armor from mesh armor (beyond simply an increased "armor value"), which is a functional distinction in reality.
You could also add that pierce attacks do increased damage against the "mesh" armors (Mail is decent protection vs. cut, but its give provides little protection against a piercing attack). If however, the game already simulates this adequately, then it is not worth changing.
I agree, with what your saying (especially about pierce weapons too, and if the math already simulates this, then there would be no need to change.) But I dont think it does simulate this currently...
You could also make 'chances' to hit the chain in between the plate pieces... Like 25% on transitional 10% on the milanese etc...
Which brings me to another point - doing the same thing for other armor types, and damage types...
So for mesh, you could make the -cut modifier like -4 or something, and chain -3...
And for pierce and blunt? well, they are better for damage for heavier armor, but they have thier limitations as well... Maybe -3 for pierce and blunt against plate, -2 for pierce -1 for blunt against mesh, and -0 against chain and below for pierce and blunt, unless you hit that 25% vs transitional or 10% versus full plate, where again, these damage types would do full damage/chain mail modifier damage (simulating hitting the chain between the plate pieces) against the plate.