He has been better, but I think Rhaast is still super strong with the right items.

I like the attempt to code, it's double [ followed by another dobule ] ending up with Black Cleaver.

So onto your question. BC being weak. I disagree because I'm usually going to be taking a lot of damage in either form and a 400 HP buffer seems to be perfect to survive. It's also 500g cheaper than a Death's Dance for the early game, offers the most offencive stats out of any HP item (such as Titanic Hydra, Frozen Mallet and Sterak's Gage. Whilst they might have more AD or more HP, the rage and armour shred passives add a LOT of damage into combo's. In combination with lethality you'll be penetrating a whole lot as the armour shred will happen first getting the most out of it. Not to mention if the enemy team even has 1 tank it'll grow in value obscenely quickly.

However there are 2 things I didn't really cover in the guide (mainly because after this patch it would of been a wasted effort) and that is the power of Duskblade of Draktharr and it's place when snowballing.

I want to emphasise, I would never skip black cleaver. If I don't get it second, it will be third or fourth. Stats + CDR + passives are way too good.

If you're snowballing pretty handily (such as you're 5/2/2 and the squishies on the enemy team are 2-3 levels below you) at around the 17-21 min mark then you can delay your black cleaver and buy a Duskblade. As you can imagine, and probably have done in your games, is noticed the duskblade passive doing work.

The sweeper (blackout) passive is great for snowballing a lead by letting you get picks that would otherwise be warded and of course the OP bonus damage upon being seen for the first time. Together they'll make Kayn do a lot of damage and move around the map freely.

However, as a general rule of thumb you won't be snowballing and sadly Kayn isn't the best champion to abuse Duskblade. Champions that can reset during a fight really abuse it, and sadly Umbral Trespass doesn't cause the passive to reset making it a one and done thing. Kha'Zix, Vayne (lol), Talon, Shaco and Wukong are real abusers of it. Every champ that buy's lethality it's good on (including Kayn) but unless you have invis in your kit then it's a good snowballing item rather than a must buy 100%.

TLDR: Duskblade is good, infact it's great. However Black cleaver is also great. DB for snowballing, BC otherwise. DB is great on champions who can use it's passive more than once, Kayn is not one of those champions so it's just "alright" in comparison.