Honey, You've Got a Big Storm Comin'

They could also be for practical reasons, job-related or other. Albatross probably needs them a lot while dealing with vehicles, to keep light and detritus out of his eyes. If he always has them on, he's always ready to go. If he always has them on, even when it's dark out, he's gradually making himself more and more sensitive to light, may become too light-sensitive to go without them, and, if he keeps it up for a really long time, may end up with permanent eye damage. Albatross could have migraine-related, medication-related, or other light sensitivity; if he does, it's also symbolic or writer-related because it has no influence on the plot. A mafioso gradually becoming more and more accustomed to the darkness until he can't handle looking at light could fit the darkness/light symbolism in BSD and/or in Baudelaire's writing

Speaking of darkness/light symbolism, the most glaring thing about the sunglasses is that they hide his eyes and Harukawa is notoriously fond of symbolic eye shading. She's stated that the more "tainted" a character is, the darker their eyes tend to be; PM characters tend to have larger pupils; and characters who can still return to the good side tend to have light in their eyes more. Chuuya's eyes in his 15 and SB portraits are dark with pinpricks of light for pupils which are gone by the main era (but he has light in his eyes in some manga panels), Iceman's totally dark, Doc's light with large dark vertical pupils, Lippmann's rather light, Piano Man's partway between Lippmann's and Doc's, Verlaine's surprisingly light but with odd pupils, Adam's unsurprisingly light and with odd pupils, etc. Behind her sunglasses, Higuchi's eyes are as huge and bright as Kenji's; Kajii's, behind his semitransparent goggles, are totally dark. Albatross', though? Who knows? Maybe we're supposed to wonder

There are a few reasons he might want to hide his eyes. He could be trying to make himself harder to read, hiding eye trauma, hiding the color for some reason (self-consciousness? to make himself harder to identify?) or, as is commonly assumed about those wearing sunglasses at night, hiding signs of substance use. Chuuya says, with possible hyperbole, that Albatross goes on nightly benders; Baudelaire wrote extensively about alcohol, opium and hashish; and whether it was Asagiri or Hoshikawa's choice, Albatross is seen actively drinking the most at the party

As for the Baudelaire angle, I've looked and looked in Les Fleurs du mal and while eyes are mentioned over seventy times in the 1861 edition, which seems like an unusually high number even considering how common eyes are in poetry, nothing stands out as a clear possible inspiration. The eyes there tend to be burning, full of tears (which one might smile through), alluring, fatal, hollow, deep, dark, and/or green and are seldom hidden. I have yet to read most of Le Spleen de Paris and have read next to nothing from Baudelaire's other works, so I may be missing something obvious and major from one of them or overlooking something in LFM

The most potentially relevant-seeming bits in LFM include eyes whose color can't be determined which seem as if hidden by a vapor in "Ciel brouillé", "eyes where nothing is revealed / [o]f bitter or sweet, / ...two cold jewels where are mingled / [i]ron and gold" in "Le Serpent qui danse" (William Aggeler translation), consumed/burned-out eyes in "Les Plaintes d'un Icare", eyes like grottos where treasures sparkle behind shadows in “Les Yeux de Berthe”, a jet-eyed statue comparison in "Je te donne ces vers afin que si mon nom", hollow eyes in several poems including "La Muse malade", a light which mortal eyes are but sad mirrors for in "Bénédiction", a woman with polished eyes made of minerals in "Avec ses vêtements ondoyants et nacrés", the abysses Death personified has where eyes would be in "Danse macabre", eyes of mixed metal and agate in "Le Chat (Viens, mon beau chat)", and pure mirror eyes in "La Beauté". Most of those poems don't seem terribly Albatross-relevant overall and Albatross taking his appearance partly from a cat or the personification of beauty seems especially unlikely but you never know

Seeing imaginary or hallucinatory things in the darkness, including nonexistent/faraway locales, is a motif in LFM, though the only decent examples I can think of at the moment are in "La Voix", "Le Voyage", and "Obsession". The first two are pretty Albatross-ish. Beyond that, there's so much darkness and light symbolism that separating any of it out as more or less likely to be relevant would be a tall task

In IRL Rimbaud's famous "Lettre du Voyant" ("Letter of the Seer"), he calls Baudelaire "the first seer" but also says that he lived in too artistic a milieu and his forms were petty/not novel enough, which could be why Albatross' eyes have attention drawn to them via their hiddenness, though that seems like a stretch

Whether it was a coincidence or not, I find it compelling that Baudelaire was said to have a mostly impassive face but highly expressive eyes, which were one of his main means of communication after a stroke took away his ability to say anything other than "crénom", and used a lot of eye symbolism in his poetry, whereas Albatross often has a cheerful look on his face and is very expressive but hides his eyes and loses the ability to see before he dies

...

I think a few of these could make sense. There are likely other logical explanations, and even if the truth were that Asagiri took the burning fatal eyes thing literally and Albatross wears shades to spare the world from his Medusa lasers, we’ll probably never know