Hey, just thought I'd type this out so that it'd exist somewhere outside of my own head:

You know how Bellina's skin is a slightly-shiny silvery-grey, and her hair and eyes (including sclera) are blue? And you know how when she takes on a Passenger, she gets weird darker-grey spiky/geometric designs on her skin and her eyes shift from blue to red?

Well, while all of this is (of course) mediated by her magical pact (and thus can't be changed, or at least, not easily), it does still have to have a physical manifestation of some sort, it isn't just illusion. Specifically, Bellina's body has a complex chemical compound called bellinium that takes the place of melanin, and performs a similar function, but for magical energy. Each molecule of this pigment is in the shape of a nanoscopic pentagon, with atoms of silver, quicksilver, cobalt, aluminum, and platinum (surrounded by elaborate nests of hydrogen and oxygen atoms, of course) at each of the five corners of that pentagon.

Because these molecules were designed with intention and take a polygonal form, each one basically acts as a tiny ward/glyph/summoning circle, taking in ambient magical energy from Bellina's environment in much the same way that melanin takes in UV light - Bellina Nyteshade's body is a passive magic sink. This energy actually helps to reinforce the various enchantments and bonds that perpetuate her physical form, and, in particularly high-magic environments, also boosts her metabolism, mental acuity, and casting ability. Yes, this also means that most spells cast on her, whether positive or negative, do have a slightly reduced effect, or may require more power to work at all. It also means that minor tokens of bad luck (broken mirrors, black cats, etc) actually are effectively good luck for her, because her body simply absorbs them, without her even needing to think about it!

Bellinium makes Bellina's skin cells look grey everywhere she would normally have melanin, which is why her mucous membranes are still pink. This same molecule is in her hair and eyes, where it displays in a slightly altered form - the metal atoms take on properties more like their oxides, hence the cobalt blue.

Unlike melanin, bellinium does not accumulate in response to absorbing more energy - she stays the same level of grey all the time... except when that Passenger is aboard! Because the magic required to maintain the bond with her pact patron is more focused and specific, it flows through her in various geometric patterns that are usually - but not always - symmetrical along her body's bilateral axis, sort of like inside-out runic tan lines. This isn't new bellinium - it pulls the excess out of her irises and sclera, which is why they show as red and pink - that isn't some sort of weird blood magic and it isn't meant to indicate anything about the attitude or moral alignment of the Passenger, that's just what colour her eyes are when you remove the pigment, in much the same way that albino animals usually have red or pink eyes.

Now you may be saying - Tailsteak, that sounds really fiddly and scientific, that isn't Bellina's vibe! And you'd be right. Wanda Wix helped to develop the compound... though she did so after meeting Bellina.

You see, Bellina didn't include bellinium in her original spec diagrams for the body she wanted, but she didn't not include it, and some of her patron entities are asynchronous relative to her, meaning that they were able to see Wanda inventing the molecule, Bellina agreeing to it, and then (as far as they're concerned, in a normal linear fashion) applying that knowledge to her pact as it first manifested. From Bellina and Wanda's perspective, Wanda looked at Bellina, guessed how her skin worked, invented the molecule as a hypothesis, Bellina agreed that bellinium was a good idea, then they examined her epidermis (hard to do, since she doesn't shed skin cells), and confirmed that Wanda had been correct all along.

This doesn't really change anything about Bellina or how she operates, I just thought you might like to know.