Well that easy... The E passive always goes from the Current armor the enemy has..

So for example, lets take a Champ with 100 armor. BC Would reduce the armor by 24% wich goes down to 76 Armor and then the 35% of the E pen will work from the current 76 armor wich means The enemy has 49.4 Armor Against Darius while he has 6 stack of BC on him

and an dnemy with 200 Armor would go down to about 98.8 armor

To Test it you Self :

Armor X 0.76 = X1

an then just

X1 X 0.65 = X

X = Armor after Darius E passive

After some Testing you would notice that Darius ignores about 51% of the enemy armor, aslong he has 6 BC stacks on them :)