As per the title - with the nerf to Hubris, I think Bastionbreaker may be the best first item for Kayn to ensure a snowball. And I'd argue that for Blue Kayn, the follow-up should likely be Axiom Arc almost every time.
Here's the pertinent patch notes:
Hubris:
Base AD: 60 -> 55
Passive AD per Stack: +2 -> +3
Bonus AD: +15 -> +12
Axiom Arc:
Passive: Refunds 15% (+.15% per Lethality) -> Refunds 10% (+.25% per Lethality)
Bastionbreaker gives 55 AD, 22 Lethality, and 15 Haste. That's as much AD as new Hubris, but more Lethality and Haste. It actually gives the most base Lethality of any item.
With Axiom Arc as the second item after Hubris, we used to get 15% (+.15 x 38) = 20.7%. Now with Bastionbreaker we'd get 10%(+.25 x 40) = 20%.
We also get a passive that works great to get extra gold off every gank. We can push Lane fast, so at one item if we get a good gank off, we now often also get a turret. Plus, it adds True Damage to the initial Q or W on a gank - 30+1.5 per Lethality. so 63 True Damage at the start of a gank which isn't awful.
Obviously it's too soon to know if it's the best First Item for Blue Kayn, but I think it deserves more experimentation now that Axiom and Hubris are both pretty meh to start. I'd say that Voltaic is likely the main competition, since it's been changed to be very Assassin friendly.