I think Zephyr would be a good replacement for Black Cleaver on Darius situationally

When fighting, Darius wants to trigger Noxian Might as soon as possible which takes a while given his low attack speed and I think Zephyr would be a solid alternative for Black Cleaver against teams that have a lighter frontline or when your team is dealing primarily magic damage.

Black Cleaver gives 400 health. 40 attack damage, 20% cdr, armor shred and MS on hit (20 for hitting something, 60 for killing something).

Zephyr on the other hand gives 50% attack speed, 25 attack damage, 10% cdr, 10% MS (which is gonna give about 40 flat ms after buying tier 2 boots) as well as tenacity.

So Zephyr allows you to apply bleed stacks much quicker, is better at chasing when not in combat (or even when you have bleed applied to someone), and gives tenacity which allows you to buy ninja tabi against heavy autoattackers or boots of swiftness for more ms.

Obviously you don't get as much health, ad and cdr but oftentimes a teamfight can be decided by how quickly Darius can activate Noxian Might and start dunking targets so I think it should be considered as a viable first item for darius.