I know probably everyone figured this thing out already but for those that didn't think much about it here is my theory on every act's name and meaning.
Huge spoiler alert for Fontaine's archon quest.
Act I
Prelude of Blancheur and Noirceur.
Blanc is white in french and noir is black in french.
Prelude means something that happened before the main event (I may be wrong but anyways).
If we add em up we get a black and white prequel,which is quite fitting.
Reasons:
By black and white it means that this AQ it's going to be more tragic and mature than the ones before it,which fits because in the very first act two people died (one crushed and the other evaporated) which doesn't happen that often in genshin impact (at least in the main quests).
By Prequel it means that it's not really the most important part in the story and there'll be more important one in the future,which it's fitting because the traveler could've just ignored all the court thing and the outcome wouldn't change that much,another reason is that's the very first act.
Act II
As light rain falls without reason.
This one is obviously related with the hydro dragon/Neuvillette because whenever he's sad,light rain falls,which does fit the most with act II
Reasons:
Navia and her father's story seemed to have touched Neuvillette as he said at the end:
"He sacrificed himself for justice,is there any purer form of justice than this?" (Or something similar) and light rain fell multiple times in this act.
Act III
To the stars shining in the depths.
First off it's obviously related to the fortress of meropide being deep underwater and "to the" means that the protagonist goes there,the stars shining might point out the bright mind of Wriothesley (he's way too positive for a jail leader) and the "bright" atmosphere in the fortress.
However shining alone could also point out the appearance of the primordial sea.
Act IV
Cataclysm's quickening.
This is related with the fact that the danger is getting dangerously close as the primordial sea almost got out of control and would've dissolved everyone if not for Wrio and Neuvi resolving it momentarily.
Cataclysm means an "end of the world event" which makes sense because the primordial sea can dissolve all of Fontaine's life.
Quickening means something that's approaching rapidly.
Act V
Masquerade of the Guilty.
This is related with Furina's 500 year long masquerade (masquerade means putting up an act covering the truth) of hiding the truth that she is just a normal human put in the archon's chair so the real archon can do her act of giving the power back to the dragon sovereign.
Guilty points out to how Fontaine's population is "guilty" of existing because Egeria created them from oceanids without Celestia's approval.
Furina is a human too so she's guilty as well
That's all
Thank you for reading and your attention.
I might've thought about it a bit too much.
Anyways this is just my theory.