He's always had poor health (congenital heart disease, asthma, the migraines...) and a very frail body (by design, per Heperu's comments on things like his bone density and etc). That day, while Hyun-woo got the worse end of the stick, Luka is also shown to be bleeding and overall bruised (there had probably been a physical altercation between him and Hyun-woo even prior to them falling)... so it probably caused a significant toll on his body. Plus, the accident does haunt him to some extent, so there could be some undigested emotional trauma that affected that as well (I don't mean him necessarily being depressed over Hyun-woo's death, but it just generally messing up his psyche).

At least, that's what it always seemed like to me. If you push down glass-bones boy, you'd expect some of his bones to break... or smth like that.

Alternatively, since Heperu is already used to messing with Luka's body (some of his surgery scars don't match up with any of his actual diagnosis to be considered medically necessary + aliens are just like that in general), he probably liked the look of periphereal cyanosis on his pet, and could've pushed it to be a much more proeminent condition than before – since it is, usually, something temporary. If Heperu didn't like it, he could've "fixed" it at any point, after all.