Isn’t that when someone identify themselves and their blog as a fictional character from fictional world? Here’s what I found on it.
Fictionkin, also known as Mediakin and Otakukin and rarely Otakin are members of the otherkin community whose identity as otherkin stems from something generally considered fictional: specific characters, invented species, or fictional worlds. An otherkin identifying as Legolas from Tolkien’s Lord of the Rings would qualify as Fictionkin, as would someone claiming to be an Ecthros from Madeline L’Engle’s works, even if that specific Ecthros was never mentioned in the books.
I’m not well verse with this but I’m pretty sure it’s similar to roleplay blogs but instead of portraying a character the person sort of becomes the fiction, right? Sorry if this isn’t helpful! But that’s all I can find on it.