So I have a lot of OCs! Not as many as some people, but still a fair few, and I kinda tend to sort them into big families, or 'bloodlines'. They tend to have weirdly coloured blood because I really like that trope lol
I wanted to make infographics on most of those bloodlines for fun, so I might do this for all of them, but I started with this one since it's important to some of my main OCs, like Viscon or Elec.
...It also kinda explains why her eye looks like that, which is a question that gets asked sometimes.
So if you're not interested in my lore, I just hope you like the quick doodles I made, and if you're interested, I hope you like this huge wall of words I posted under 8D

Dakon's bio that has the same info: https://toyhou.se/1776694.dakon

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Brief History

First specimens were created millennia ago by Dakon's influence, and they had the form of Mobian hedgehogs with crystalline spikes, diamond-shaped markings, toxic abilities and, obviously, black blood and flesh. In modern times, most common species of black-blooded demon Mobians are still hedgehogs and cats, but over the millennia there have been demon individuals in most recognised species on Mobius, with a wide range of powers and looks.

Demon/Mobian forms and genetics

The demonic form of these Mobians is always a small-sized, cat-like creature with a tail ending in a scorpion-like, crystalline pincer. They have the ability to secrete a liquid, sometimes called essence. The essence is a thick, slightly fluorescent, and very poisonous liquid, capable of killing a regular Mobian within a minute. It can solidify into crystals at the demon's will, and it often looks like they can sprout crystals from under their fur. The demon and Mobian forms can both materialise visual organs anywhere on their body.
It's important to note that in 99% of the cases, the main eye/essence/crystal colour is the exact same.
Over the years of course, the black-blooded demon genes got mixed with genes of other species of demons, angels, mobians and everything else. Nowadays, a lot of hybrids have demon forms sharing traits of both species, and many black-blooded Mobians no longer share some traits with the ancestor. In general, the more traits a Mobian shares with the ancestor, the more 'pure-blooded' they can be (though it's also not a necessity, as seen with Blackbow family). Similarly to vampires and other such supernatural creatures, black-blooded Mobians can sustain themselves on blood or flesh or souls, but it's not necessary for their survival, and obviously nowadays is barely practiced. In fact, trying any of these is ill-advised, as the demon can develop an addiction which can be very problematic to fight off.

Overflow Condition

The closer to pure blood a black-blooded demon is, the more likely it is he or she will eventually suffer from overflow. Trying to sustain yourself on Mobians can also increase chances of future overflow.
Generally harmless and non-lethal, overflow is still a very uncomfortable condition. When a black-blooded Mobian doesn't use his demon abilities or change into his demon form for a long time, the essence can build up in his body. Eventually it reaches a critical point, and the divide between the Mobian and demon forms temporarily becomes blurred. The afflicted Mobian can't control the essence, and it involuntarily pours out of orifices. With the essence pouring over the Mobian's body, it can also open up additional vision organs, and despite the ears and eyes of the Mobian being clogged up, their senses are enhanced.
The overflow can last anywhere from a few days to a few months. It doesn't require hospitalization, but can be very uncomfortable and sometimes painful (during additional eyes opening up for example). The essence also stains horribly and is hard to wash out.
Not every black-blooded individual has to experience overflow in their life, it's only made more possible by conditions mentioned before.

Examples of black-blooded Mobians

Electrisa
Traits: cat-like demon form (doesn't use it), eye manifestation (doesn't use it voluntarily), enhanced senses, notable essence influence
Due to her Mobian form being permanently damaged by holy magic, the demon form partially merged with the Mobian form, causing her left eye to appear as an ever-shifting puddle of essence. It also has a tendency to drip out of her eyesocket, especially when crying. Despite its looks, it's still a functional eye - in fact, it's more light-sensitive than Elec's right eye.
Worth noting is that the event also caused damage to her demon form, although she doesn't use it. If Elec were to turn into her cat-like form, it'd appear to have a chunk of its head missing, with essence pouring out of it in an endless stream.

Haspar Blackbow
Traits: Vision-related powers, multiple optical organs (permanent).
An example of a hybrid demon, Haspar (and his many siblings) have demon forms whose looks are related to their other demon ancestor, not Dakon. They're all monochrome, and definitely don't look like cats. Despite that, they all have vision-related powers, and although their essence isn't visible, it'd be green like their eyes. Genetically, they are very close to Dakon (only 3 generations apart), but at first glance don't share that many traits with him, and take after their second demon ancestor instead.