Mori is strictly a Japanese Fashion style. As a Japanese style, it originated with and was predominantly molded by a woman named Choco. She has written several books about the style (though I’m unaware of English translations at this point), created the first online community dedicated to the style (also in Japanese), and wrote the official Mori Manifesto. You can view them all respectively at their links; alternatively here’s a “Guide” that gives you a bit of the same brief history while also going in more depth about fabrics, patterns, etc.
In terms of Mori vs. Dark Mori, what little actual explanation I have managed to find on the Dark Mori style and history points to some interesting things. Chief among those finds, however, is the fact that true Dark Mori actually utilizes the same patterns, cuts, silhouettes, colors, etc, (aka: Rules) of your standard Mori- with the exception of Creams, whites, and similar colors being replaced by greys, blacks, and related ones instead. In other words, for all intents and purposes, Dark Mori should not look much different than Mori except in regards to its base color schemes.
This leads me to believe that what most Westerners have been practicing is heavily removed from the actual Dark Mori style as it’s meant to look- which could have lent a large helping hand to the creation of Strega; Strega utilizes common westernized ideals of Goth fashion to some regard- which actually should not be present in Dark Mori but has still somehow managed to weasel its way in… More than likely due to Westernized need to make anything utilizing black stereotypically ”Goth”.
This is relevant to Strega, however, because Strega has roots in the Japanese Mori and Hama styles and follows a relatively similar aesthetic to Dark Mori. However, Strega is not a Japanese fashion style- nor is it a member of the Mori fashion family. Instead it is strictly considered a European one, originating with European lovers of Mori who didn’t like the strict rules Mori fashion follows, and didn’t feel comfortable calling what they did Mori; the result was (to my understanding) to combine the aesthetic of Witches with the overall aesthetic of Westernized Dak Mori and other similar styles into a new style.
Strega as it is emerging follows a very simple Fashion Manifesto. Currently this is the only known and even remote authority on Strega at the moment. This manifesto states- very clearly- that Strega is not related to the Mori family of fashion due to it not following the rules. In fact, Strega doesn’t have any rules except 1: That is be inspired by Witches, Witchcraft, Folk and Faery tales, and similar things. Other than that, it was only roughly inspired by Westernized Dark Mori and most people who currently practice it do or originally did wear or have an interest in Mori in some form.
History: Two possible originators.
The first is the Tumbr user StregasForest. According to this user, they were the first ones to originally think of the term. However, they used it to refer specifically to their particular brand of Witchy inspired Dark Mori, and did not consider Strega to be anything more than another term for Dark Mori.
The issue with this arises from the fact that Dark Mori is potentially being practiced incorrectly as their band of “Dark Mori” does not follow the correct and traditional inspiration for such style. Instead the style closely resembles a more Mori-influenced Strega and one cannot deny the similarities between this user’s “Dark Mori Checklist” and Strega’s inspirational base written into the Manifesto.
The other potential creator is the Tumblr user ShortCutToTheStars. You can kind of see the origins post here, though I haven’t found the original conversation that led to it. This post by the other user mentioned, however, alludes to the fact that there was a question answered somewhere by the user that sparked it all.
The “Strega Fashion Manifesto” outlines a lot of the basics- such as there being no real rules to it so long as it’s Witch and Fairytale / Folklore inspired. The same user who posted the Manifesto has a F&Q and if you sift through there there’s more in depth answers such as this one giving a brief (and yet more in depth) answer explaining the differences between Mori and Strega, and its origins. Likewise, they have more posts in their Strega tag on their blog.
This is, to my knowledge, the first instance of its mention as becoming an official-ish fashion style instead of it being considered a Mori sub-group. Mostly, however, it seems to be confined to Tumblr users.