Black Cleaver is outperforming

Black cleaver is outperforming every other option (rageblade, last whisper variants) when it comes to damage (damage reduc. on the full armor), coupled with the stats it comes loaded with. Ordinarily you'd say 'but autoattackers don't have any ways of doing multiple hits!' Except they do, especially with passive procs (titanic, bork, etc.) from various damage sources and devourer coupled with the tightening up in various kits.

Several possibilities come to mind:

Reduced damage reduction and receive a flat ad nerf but a health buff

Changing the ratio to take more hits

Alternatively tinker with the mechanics and count damage passives that strike on-hit as part of the 'basic attack damage' e.g., a single hit (actives not withstanding).

...or some combination