I was able to figure out where some strange claim I saw long ago from someone complaining about Marvel and saying Loki can't shapeshift and the gods depend on animal skins to do so came from (despite this contradicting the myths). It came from non-Marvel norse mythology inspired comics Valhalla by Peter Madsen (which are popular in Scandinavia).
I found out because I was curious as to how they managed to make some myths "kid-friendly" and got my answer about how shapeshifting works i those comics through that.
So in Valhalla series ( ie the wonderful Danish comic adaptation series of Norse Myths ), Sigyn is a volva who came from a seidr tribe in Asgard. Her father is an Aesir, her mother a Light Fae. And she was at first part of the Aesir Seidr order and a handmaid of Frigg....and then she caught the eye of Loki and the rest of history
Valhalla!Sigyn gives me vibes of Luna Lovegood and Helga from Vikings in spades - she got big sea green eyes, tanned skin, freckled cheeks and thick curly dark blonde hair
MCU Loki (properly outfitted in Avengers-era or Thor-era glory to remind/haunt us with What Once Was ... a.k.a. you HIDE complicated origins from miette? you kick their popularity down the field like the football? oh! oh! even in space jail! AU jails for centuries!!!!)
Because the AI user that just annoyed me also claimed myth!Loki's canonical hair color is blonde and that's why they did that crappy blonde MCU!Loki AI art, and I am feeling spiteful cus fuck AI and that eyesore, and I need to wash that off my brain.
AND I KNOW this claim came from those people that interact with THAT ONE VERY obnoxious Sigyn stan that pretends to be a Norse mythology expert despite never providing proper citations or citing modern myth re-tellings and whines about children's story books not being accurate. Ask her to cite her claims and actually look at the sources, for the love of Loki!
LOKI HAS NO KNOWN HAIR COLOR IN THE MYTHS! THERE's NO SOURCE FOR THAT! Loki's also a shape-shifter! Loki can have their hair look like whatever the fuck they want. To my knowledge, the only gods that have confirmed hair colors are myth!Thor, who is said to a red-head, and myth!Sif being a blonde who gets a wig of magical gold to substitute her chorn hair. There's also a description of myth!Heimdall who is said to be the "whitest" god, implying blonde/white hair? Loki's only mentioned to be pretty, assumed to be Aesir -sized rather than gigantic, likely smaller than myth!Thor because myth!Thor can kick his ass and he can hold onto Thor's belt when crossing a river (Skáldskaparmál), and have scars on his lips due to them being sown shut.
Also from Gylfanning: "Loki is beautiful and comely to look upon, evil in spirit., very fickle in habit. He surpassed other men in that wisdom which is called 'sleight,' and had artifices for all occasions; he would ever bring the Æsir into great hardships, and then get them out with crafty counsel."
There's also the Snaptun stone, believed to be Loki due to the scarred lips, giving him a mustache.
The Viking Age Snaptun stone was carved around 1000 CE and shows a face with stitched-up lips, which reminds of a story preserved in the Pro
The most common and widespread hair color depictions of Loki have red/ginger/orangey hair due to Loki mistakenly being thought to associated with fire because their name's too similar to the fire personification Jotun's name, Logi, because Richard Wagner fucked up in his Ring Cycle Operas and combined the two of them into "Loge". Arthur Rackham's illustrations based on Wagner's Ring Cycle are possibly the most influential in that regard because they're fucking awesome. Even GOW made their Loki depiction a red-head.
The Rhine maidens ask for the help of God Loki is one of artworks by Arthur Rackham. Artwork analysis, large resolution images, user comment
And here's a painting that looks like a ginger to me by James Doyle Penrose (1912):
The second most common hair color depiction is black hair, probably thanks to Marvel as it's most prevalent in modern depictions (e.g. Alan Cumming's Loki in "Son of the Mask", whom I fucking love).
There's also the Valhalla comics that started in 1979 (after Marvel, but I don't think that contributed AT ALL because Vallhala comics are way more myth-accurate and fucking awesome. I am just including it here because it's a black-haired Loki depiction).
There's also Eric M. Esquivel's Ragnarok n' Roll (have not read and I am not inclined to do so, but it kept showing up in my loki searches 10+ years ago and its existence was archived in my brain due to that). Is it me or does he looks like Cantinflas because of the mustache?
This is Cantinflas btw
There's also Jul I Valhall Loke (Thanks Scandinavian Twitter users for showing me this gem even if I can't understand the language. I am forever grateful.)
And surprisingly, Marvel was not the first to give him black hair. Marten Eskil Winge portrayed Loki with black hair in his 1863 painting "Loki and Sigyn."
‘Loki And Sigyn’ was created in 1863 by Mårten Eskil Winge in Romanticism style. Find more prominent pieces of mythological painting at Wiki
Neil Gaiman has depicted Loki with a variety of hair colors, USUALLY as a literally "fiery" red-head (Sandman comics & American Gods book), whatever Johnathan Tucker's hair color is in the American Gods TV show, and black-haired (Norse Mythology graphic novel).
There's a brown-haired Loki depiction in Assassin's Creed (I have not played it btw).
And sometimes he's just some demonic thing with horns:
