Ink is not-reality outside of a certain world. Its yawning hunger for being is subtle, it is seeping into the world in deep and cracked places, coming to light through the long roots of plants, going behind our eyes as we sleep; it flows from pens of writers and brushes of artists, takes the sound of oracular rhythms, intertwines itself with the veins of stone hewed from deep quarries. One day Ink will extinguish the sun to add its amber husk to the necklace of devoured worlds but the pace of doom is slow and full of beauty.
By the long chain of gradual becoming – through the cracks, through the waters, through the roots – Ink can be rendered from certain desert plants and obscured minerals into atrament of liquid, tangible not-reality. Without an intend it remains inert. But such intent is easy to find.
Once I played 'Dishonored', 'Mark of the Ninja' and 'Incredible Adventures of Van Helsing' in quick succession, and the concept of Ink gestalted out of them, despite all three games having only few elements in common. 'Van Helsing' contributed the name, the theme of creativity and the idea of beauty, 'Dishonored' the aspects of hunger, melody, dead worlds and light-bearing beasts, 'Mark of the Ninja' gave the ideas of toxicity to the user, turning into other kind of being, strictly no-direct-damage powers, of silence and of pathways into reality. 'Dishonored' and 'Mark' were both stealth/assassin games with tattoos on main protagonists, 'Dishonored' and 'Van Helsing' both were quasi-Victorian in their tech/fashion sense, had a sort of Outsider being, and had dark-blue voids full of scattered fragments of no-worlds, but the only connection between 'Mark' and 'Van Helsing' is that they are both in third-person perspective, have a female sidekick, and a madman or two. Later 'Adam Wake' and its concept of Darkness as both associated with creativity, coming into being and deep waters just solidified the whole thing.
Ink can serve as plane of shadows although the initial idea that this isn't a shadow of the world, like PoS is, but what the world isn't.
Another thing is that Ink is not supposed to be always available: in most of the magics, once the access is established (spells, MPs, etc), it is always here and regenerates automatically over time once rest conditions are made and unless in a specific dead magic zone. I see Ink in its initial form of atrament to be scarce in a way cochineal was initially scarce – it was taken a lot of effort and resources to produce a drop of paint, and once it was used it was gone. Tattoo and charms are workaround for such approach, but with the tradeout in faster erosion of self for the mage.
On another note I always wanted to have no-direct-damage magic, the magic that is tied to tangible things that are still easy enough to manage but not common and broad enough to be commodity, the magic that creates not only grotesque and dangerous but also beautiful things, the magic with limitations built into its nature/theme, the magic that comes with the cost to both the wielder and on the grander scheme to the world, but not as drastic and random cost as spell mutation and spell catastrophes – more like a gradual transformation/erosion proportional to mage's ambition, and overall sense of faraway but unavoidable doom. For this purpose alchemy never worked for me despite its tangible basis because the appearance/trope of alchemy in modern perception has no limitations (science+magic), is a step away from magitech, is detachable from the wielder without much trouble, and its material components are so broad and varied that they are very often either abstracted or handwaved; also alchemy has way too many explosions.
Spells as spiritual beings that live in the mage's brain idea comes with the cost to the wielder and skulls of mages that are carved by such spells into the 'spellbooks' or charms, which is great, but otherwise it has no innate theme, and from what I've seen spells as beings have little weight or character on its own. Of course, Ink can also be stretched to have-everything-all-the-time type of magic, with inkblast, ink black tentacles and inkbombs, making it into uniform characterless magic just like everything else, but at least for myself I envision its 'step-by-step gradual becoming into reality through intention of real people' origin as preventing this. The bottle of ink can stain like any physical object but without intention of the wielder using it to draw or write it won't become anything more substantial or meaningful.
No-direct-damage idea also stems from the games (even rats and bees in 'Dishonored', which come closest to be the direct attack, are technically summons), and to distinguish the caster more from a warrior; I was also bored by 5ed spells, I think, and wanted the mage to rely more on indirect or subtle means of dealing with problems.
Idea of Ink creatures being beautiful is like idea of sky: it is a natural phenomenon that is perceived as beautiful by us despite its many appearances but is indifferent to our understanding/need for beauty. Unlike fae, angels and demons, kind-eyed are beautiful not to entice, seduce, intimidate or awe us, and it is more of coincidence that they are perceived as such.
And Ink also easily bridges with Colour, the parallel and opposite principle, if needed.
(1) via pods5.com
(2) via netluxury.com
(3) by visothkakvei
(4) via Rainbow Six Siege