I think there are a number of reasons--
1. Rights Issues: Black Vulcan; Juice; Soul Power/Sparky
2. Public Consciousness: Black Vulcan is the first Black superhero a lot of Boomers saw, which later influenced their work. With regards to the anime/manga characters, the creators likely looked up black superheroes, though the idea of a Lightning Ninja was hinted at way before these characters showed up... Maybe it was a generic idea until they caught on to the commonality, which generated ideas for real characters. Naruto is one of the first big anime Ive seen go out of their way to show black characters, even though there are tons of Black anime fans.
3. Running Gag: At some point someone noticed, and like Hollywood's Wilhelm scream it has become a running gag. See Volt. Now we're seeing white electric heroes race bent into black ones to keep the gag going. Running gag characters are also likely influenced by Storm.
[Side note: There are also lots of Black speedsters who started off as parody characters. I guess someone thought the Fastest Man Alive title should be a black guy or maybe it's a comment on black celebrity athletes or both. Keep an eye out for a trend in Black mer-people with Disney's racebending of Ariel in The Little Mermaid.]
4. Design/Homage: Static
5. Possibly Accidental: Miles Morale's sting may not have originally been electric-based, but artists like Sara Pichelli used abstract bolt imagery to convey the idea of stinging pain. Somewhere along the way, someone saw the bolts and thought "electric." Now it is, as indicated by the Spider-verse film. Or maybe this was Bendis's intention all along, since his Miles could break out of ropes with some sort of energy blast. Aqualad's electric abilities were influenced by electric eels, so he may not have been intentional... The blond hair reminiscent of Storm's white hair may mean the creators saw the connection or it may mean they wanted to give his face an eye-catching gimmick.