armament hardening is visibly black and people talk about it in the series. it's invisible most of the time but if you see someone turn something black, everyone can generally see that. mihawk talks about how you can turn any sword black with haki to harden it and make it unbreakable.
no one has ever really commented one way or the other in-manga about haki's visibility, except for Rayleigh telling Luffy that to understand the absolute basics of armament haki, he could think of it as a suit of invisible armor.
the lightning from coc or coa or whatever it is, we know very little about the mechanics of this. any time anyone on the forum, or twitter, or reddit, or whatever, talks about how people don't see haki, or you need haki to see haki, or haki lightning is visible but hardening isnt, or hardening is only visible to the reader, or whatever, it's all headcanon. we don't actually know. it might be visible or it might not. it's probably visible because we see it in the manga and then look at people making
reaction faces about big displays of haki.
in egghead, random marines did talk about seeing the lightning. if the teleportation ability is haki, this would imply that any fodder can see the haki lightning