Family friendly children’s characters make great eldritch horrors. Garfield works perfectly because he’s an asshole character in a children’s comic. Blue is an innocent dog, so it doesn’t work as well. The more enduring route for dark fan art is more about the human characters than Blue.

It’d be like turning Hobbes into a monster terrorizing Calvin. Calvin already gets chased by gross monsters in the comic, and Hobbes isn’t an innocent pacifist or a total asshole. He sometimes is a terrifying tiger with the power to hurt Calvin, but he actually loves the kid in a convincing way. It’s not like Garfield perpetuating John’s depressing life. The only way Hobbes could be scary is by holding back a maturing Calvin. Even then, the inverse is more interesting.

The story would be about getting in touch with Calvin’s imagination and growing to be a well rounded person. Calvin would be cynical and depressed, but Hobbes would rekindle his childhood imagination while not being naive. The horror could be adult and graphic, genuinely disturbing, but not mean spirited or cynical. It doesn’t need false hope or cynical nihilism. Just meditation on life. It would never happen officially, but a fan project would be great.