Seems like at one point a Darius always has to pick up a Black Cleaver. And when I play I feel that way too often. Due to the utility it gives in team fights to shred armor but also granting a 25% armor reduction ontop of his automatic 33% armor reduction from his ranked E+masteries. Makes it so he really doesn't need much for offensive items and can build more tanky.
But.. is it "needed" on Darius?

What are your thoughts about Darius and Black Cleaver. And also what do you mainly build on Darius?

Once his bleed does Physical damage it'll be core.

Do you know if they actually will be changing it to that or not?

In a way, I think the bleed being magical damage is what actually gives it is devastating bleed effect vs top laners. They build armor, not MR. And so while they build armor to negate Darius outright physical attacks, the bleed penetrates through in magic damage. In all honesty I think a bleed effect of such kind should always be this way. Internal bleeding effects are indeed penetration to the body. Which is why I believe Talon's bleed effect should be Magic Damage as well.

Once you factor in armor penetration, the bleed probably does about the same in the early game as it will if it were physical damage.

Darius magic damage is really exaggerated in most cases.

Its not for darius, its so you can press Q in teamfights and actually contribute

pretty much every AD champ builds Black cleaver now. Kog ashe cait dar sion pretty much all lol

Frozen Mallet is way better as a Darius core item than Black Cleaver. Remember that an item slow will stack multiplicativly with a spell slow. Hence, his W sticks your opponent with a 52-64% slow as you stack bleed after bleed on them. Black Cleaver is fine as a second item, tho.

Hence... it is for Darius lol.
An item that allows him to contribute to a team fight with other than just his peel skill is an item qualified for Darius.

Frozen Mallet is a nice item to get indeed.