Blunt damage numbers are typically lower. Black armour is something like 26. A club does 11b. So what if it bypases 1/2 the armour? 11<13. Hence, 0 damage.
Now, a falchion or something looks pretty small too but does 28 so that may actually hurt you.
As for swords like nordic swords, it doesn't do both at once. Slashes are the cutting damage and lunge/stabs are piercing.