In American English it often refers to a type of simple sweet pop music, but in British English it is used much more widely for things that are nauseatingly cute or precious. It comes from the way the word sweet sounds when said in baby talk.
Someone who is adorably sweet almost too sweet, sometimes hippie like. It's also a spin-off from the indie style they like being unique but not "in your face unique."
For all its bluster about being evil, black metal after the Norwegians is the exact opposite.
The genre welcomes any newcomer, and will immediately immerse them in a world where everyone is unique if they use the right combination of trite blasphemous cliches.
It's the only place where you can scream "I want to kill everyone and everything" and get a group hug.
You can also take Garage Band, four chords and a fifteenth-rate imitation of the truth, pop out a demo, and quickly have people calling you a genius.
Even with the imagery of violence and hatred, black metal is very sweet. The black metal Nazis don't mind if you're black, as long as you want to be a black metal Nazi.
Black metallers don't even care if you want to listen to black metal, as long as you're cool with it, they're cool with you.
It's like a big emo community, dedicated to making Satan cute and the apocalypse amusing.
But no one has yet dared use the word "twee" in public to describe it.