Pretty much the entire Castithan species in the SF series Defiance
At this point, what I’m actually finding remarkable is that there wasn’t a main character like this in Babylon 5
Or Arthas from the Warcraft universe, who even comes with his own soul-eating runeblade;
None of the aliens there had downward-facing hair.
Delenn did (after her transformation):
I’m mind-boggled that Lee Pace played every platinum blonde mentioned so far in the thread. 
Jeff Dee is cool. He’s commented on my gaming-related Facebook posts a couple times.
He definitely is. I supported a Kickstarter campaign of his, some years ago, in which he recreated art he’d done for TSR back in the '70s and '80s (all of the originals had been thrown out), to help fund the legal bills which he and his friend, Jack Herman, had run up as they had fought, in court, to regain the rights to the Villains & Vigilantes RPG (which they had created while they were in high school).
I can’t be the only one who figured we’d never see Lee Pace again after Pushing Daisies.
Not to mention the types of roles he’s been in haven’t been the ones I figured he could do after only seeing him in Pushing Daisies(and Wonderfalls, actually).
Let’s not forget this guy.
From season 2, episode 3 of “Arcane”:
Who is this guy and his family? I have no idea.
I thought they were all played by Johnny and/or Edgar Winter.
Kaecilius from Doctor Strange would be a candidate I think, but he’s got it pony-tailed.
And he’s not blonde, either (at least, not in the movie), which is part of the OP’s stated archetype.
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So, unless I’ve missed one, nobody’s found any examples from Star Wars nor Star Trek. It seems rather a stretch to me to refer to “every sci-fi fantasy movie”, when it doesn’t appear in either of the two biggest sci-fi franchises. There might also be a few in individual MCU movies, but not in most of them, either.
Fantasy, I might grant (though there are still plenty of counterexamples), given that Harry Potter and Lord of the Rings are the biggest fantasy franchises.
He’s Grey. ‘Platinum Blonde’ is sometimes a euphemism for that. Maybe.
OK, how about Ebony Maw from Doctor Strange 2
Here’s one from the Star Wars universe, but not a main character in any of the movies:
Close enough.
Avengers: Infinity War, actually. He has white hair, but I’m not easily finding a picture of him that shows the full hairstyle.
Arkanians have appeared a lot in other media (video games, comic books, etc.), but I don’t think that there’s ever been even a brief canon appearance of one in a movie or TV show.
Matthias Hues in I Come in Peace (1990, a.k.a. Dark Angel). He probably played similar roles in other films given his looks.
Maybe this?
Or artist rendition:
