Black Cleaver should only reduce your bonus armor, not total armor
I just fought a Darius with Black Cleaver and his basically handed us our asses, even when I was a 4.5k health Sejuani (and their Lucian didn't have BoRK). And in another match, the enemy had Xin Zhao, who will basically make you lose 40% of your armor if you are his target (because of his passive). Wukong? Over 50% missing armor if he hits you with his Q. The list goes on.
I dunno about you guys, but I think the new Black Cleaver is a tiny bit too strong. To remedy that, I propose that its effect should only work against bonus armor, not total armor. Essentially, it'd only work against armor from items, buffs, runes, and masteries. There, no good vs squishies or high-health/low-armor opponents.